Flashlights are good for night games or in clearing dark rooms, but as nether of those happen all that often, they're not really needed. As to giving your position away, that's what the pressure switches are for. You don't have them on all the time, you flick them on for a few secs to figure out what's around you, then release the switch and move.
You mention you're getting a magnifier, unless you have a lot of wide open ground they're not really necessary. The RDS has always done me just fine alone. But if you do get one, you should probably not go above 3x, any higher and you're going to be leaving the effective range of your gun hopelessly behind.
$450 isn't a bad price considering the upgrades, depending on what parts were used to upgrade inside. No full stock though means the battery is in the RIS (I assume) which means it's going to be a pain in the ass to get to.
A 9.6V and realcaps means you're going to have to watch the trigger, with the rof that thing likely has you'll probably be able to burn through a mag damn quick.
1700mah should be ok, especially with real caps, as long as you don't try for over-night games or anything like that. You definitely need to get a good charger though to keep it in tip-top shape.
Grenade launchers are great. That said, you want to get one, be prepared to spend easily the cost of another gun, not even mentioning the shells. The launchers range from $200 to $300 (though if you keep your eyes peeled in the classifieds then you might be able to get one for as low as $150). But that's not the real cost, the real cost is the damn shells. Cheap shells are about $30, I've learned the hard way though that when it comes to 40mm shells cheap is not the way to go. Madbulls are about the only choice. They're going to run you $60-$70 each. So a decent number of shells alone will cost you the price of a new TM.