Damn, if that TopTech is actually in good condition, that's a pretty hot price. 200 is about Combat Machine pricing, not TopTech which usually is 300-400. Make sure it actually is a TopTech (not a CM) that is in decent condition with no problems.
TopTechs are actually quite a step up from Combat Machines. The build quality of the TopTechs are also supposed to be better than the Combat Machines. I don't remember if the blowback is pneumatic or mechanical, but if it is mechanical (I think it is pneumatic though), you may want to disable it to extend the life of the rifle.
On top of that, follow naminator's good advice.
Custom VFC M4 - KJW KC02 - KJW Mk1 - TM P226 - KJW M9