Tomoe used the 'what could we do with what we have' approach. I'll use the 'what should we have them do in regard to what we ought to want to achieve' approach.
Sensors:
Sensors are both a GM's tool in offering the players of a plotship what is out there, and also a tool for the players to look out there, make the right scans and yield up useful information a GM could reward those whom know what to look for or those whom look before they leap.
Essentially, your sensors systems are composed of three fundamentals: the short-range inter-planetary sensors, the long-range interstellar sensors and finally the scanner.
Short-range sensors turn out being used by ships within the star system range. They likely have the power to give out detailed information about objects within the environment - ideally, they should be able to cover more or less all a star system, though obstruction (like something behind a planetary body) could hinder the readings obtained. Short-range sensors being in active use can make a ship easier to detect, so, ships wishing to not stand out as much can set their short-range sensors to passive mode and go in silent running. Once in silent-running, your sensors are likely only good enough to get readings on a much smaller scale (around single planets).
It's best to remember that sensors have a detection range and an identification range - these specifics are important tools both GMs and players alike will be able to use. Having sensors which are too good can be a disservice to both the plot (as the GM doesn't necessarily want the player to know everything) and the player (whom could be given the opportunity to make good-calls in using his scanning equipment to get informaion that he normally wouldn't have had without action).
Active scanning allows a ship's sensors to get a detailed scan of a specific area within its sensor range, yielding much more detail than it had previously. Active scanning would serve great use in trying to get even more information on an object in space, try to find out the presence of something trying to be elusive (silent-running vessel on the other side of a moon) or to be able to get a target lock on a target to fire weapons (usually, the other ship is aware of when you have a lock on it, so, it makes sense).
Long-range sensors are essentially the eyes of a ship beyond the immediate vicinity of a star system (greater than a 1ly range). Using those sensors, in term of combat visibility, is about equal to having a ship have the short-range sensors on active scanning mode (meaning its not discreet to anyone close by). Usually, long range sensors are more or less useful in terms of navigation since they allow most of all to be able to chart out objects within a certain range. They would likely be able to spy out something pretty showy, like a ship folding through space, at an average of 5 light years.
Active long range scanning would be able to light up any high-energy being used within an average of about 10 light years to be able to detect contacts having distinguishable power sources, or using a mean to travel which is more discreet than hyperspace - like subspace distorsion. Using active scans in such a fashion makes the scanning vessel's position really obvious, however.
Starships should yield the advantage of sensors to larger installations such has planets and starbases with superior equipment, whom could maintain watch over a much bigger area. While starships in themselves might have restricted sensor range, nothing also prohibits them from using cartography obtained from other sources, or being linked to sensors whom are in an area they cannot see and thus be able to know what is there even though their own sensors would not ordinarily be able to spy those more distant areas (that's likely how the sensor nets are made).
SUMMARY:
Short-range sensors (active): ~200 AU (+detailed scan of a 2 AU area)
Short-range sensors (standard): ~20 AU
Short-range sensors (passive): ~2 AU
Long-range sensors (active): ~10 ly
Long-range sensors (standard): ~5 ly
Long-range sensors (passive): ~2000 AU / .3 ly (navigation+cartography only)
Weaponry:
I tend not to like very long range battle which involves cruise missiles and long-distance attack since it only encourages battles to be more clinical, something which Wes seemed to want to reduce to allow players to feel more involved.
Therefore, while I can see the roleplaying value behind using a cruise missile to be shot toward a planet and then have a PC ship try to stop it, I'll focus on intra-ship combat.
Ship vs. ship is usually done at sublight speed. The fastest, most accurate weapons are usually seen as beams which go at 1c and other weapons whom go slightly less than that, from solid-ammo weaponry like railguns to accelerated particle cannons.
Regardless of a weapon's destructive potential, it's only as effective at its accuracy allows, and this hinge on its delay to impact. Typically, ships moving at 0.3c firing 0.5c to 1c fast weapons will have their engagement range go around 1 to 10 light-seconds. Any weapon being able to hit a target within that range in a reasonable delay is okay, any weapon too slow is likely only meant to be fired at something stationary or from very very close. Anything too fast is likely a cheap overpowered weapon which is way too powerful for its intended purpose in the RP.
Lasers go at 1c. If I fire from 3 light-second away (~900 000km), it's going to take 3 seconds to reach the target. In 3 seconds, the opponent likely has the time if he is aware of him being attacked to avoid being in the path of the attack. Therefore, using a laser from a closer range like 0.5 light second (~150 000km) leaves much less room to maneuver to the enemy navigator and thus helps with the attacker's ship accuracy a lot. While 1 light-second might not be the maximum firing range of a laser cannon, it does get close to the effective range the weapon itself has.
Other direct-fire weapons will follow about the same mechanic, except they aren't as fast as a beam weapon, of course.
In the case of guided warheads, they should have speeds which allow them to be seen - as Wes seems to enjoy being able to shoot down ship-killing warheads with his plotship instead of going BLARGHDEAD like how weapons such as the AS-5 and AS-7 torpedoes presently perform.
The main advantage guided warheads have over direct fire weapons is course-correction: they have a much better odds of hitting a target over long distances than direct-fire weapons since they are self-propelled. While they might not be as fast as lasers, seeing they are shortlived and unmanned they generally are able to hit higher acceleration speeds than starships (whom typically top around .3c) which means that if left unchecked, they should be able to connect with their target.
Key to making a warhead is generally making sure that it has a time-delay to impact so that the opposing ship has a chance to use point-defense weapons on it. If this is not possible, then either the weapon was used in circumstances making this impossible (which in itself should have been an opposed action, such as getting on a side of a ship which had no point defense weapons or getting close enough to insure a very short delay-to-impact) or is, like I stated before, just too powerful for its own good.