Necro noted, this is a good idea. Although lag shouldn't be too much of a problem.
Detection should also have some involvement with ship movement. A shooting star flying through the battle will be detected, but a still-sitting command center might. Perhaps a multiplier, with stealth being something you want as low as possible. Scanners should also be movement-influenced, a still-sitting scanning station/ship has a better chance of detecting something than a fast moving combat vessel.
I think they should both be block based, but take a ton of power. Large ships can afford massive scanner dishes, but a small ship probably shouldn't. Scanning/cloaking power cost could also be based on movement, so if a ship stops and waits and powers down a bit it could manage a heavier scan.
Only problem is that stations would be kinda OP with this, so stations have a base loss in stealth, and a negative multiplier in scan ability.
Numbers:
(Y is a number up to debate for balancing)
Movement-
Stealth/Scanning ability
X/(1+[% of max speed]*Y)
Power-
X*(1+[% of max speed]*Y)
Stealth based on size-
X/(surface area/Y) : Y=100?
Stealth and Scanning ability are numbers >0 that increase and decrease with added modules.
With stations, I think that having them be decent at scanning is alright. If people start spawning stations into combat, then maybe a balance change will be necessary, but until then, I think their scan would be fine. In terms of their stealth though, I agree. Stations, despite being stationary, should be easily detectable.
In terms of power however, I think with the current meta, power would be far too easy to come by, and I think a systems ratio approach right now would be better. When reactors are changed, if they ever are, then maybe a block based solution would be better.
For stealth, I think mass should be used over surface area, exclusively for creativity and performance reasons. In terms of creativity, if someone had a really large interior and a very complex ship, that would drastically raise their surface area compared to their effectiveness, so I feel like that would be somewhat unfair. For performance though, if someone's getting ripped to shit, calculating their surface area over and over again might be a little more difficult than just keeping up with their mass.
I totally agree with movement with ships though, that's a great idea!