+1 to A&K SVD
The upgrade's you need are the extended cocking handle and advanced hop unit. ASPUK are fine, but airsoftpro.cz are cheaper. A TBB will get more FPS from the spring and depending on quality may improve accuracy, but it has such a long barrel that if you get a TBB you will need to do your calculations for what spring to get, because for eg a 6.01x590mm with the stock spring will put out about 530FPS on .2g BB's. It takes AEG springs though, so it's cheap enough to experiment - an M135 is a good place to start for a 6.01mm TBB.
The stock trigger sear begins wearing out from the 1st shot, but not quickly and there is a steel one cheap enough - this begins wearing out the piston though :lol: there is a steel reinforced piston too. There is also a steel barrel base which is probably not needed unless you also do the home made mods to minimise barrel wobble and air seal leakage, but more importantly get your shooting position and technique sorted to minimise involuntary movement, but with all that sorted, the additional inertia of steel must improve accuracy. I would advise getting the steel bipod as opposed to the 'metal' one as, although a sniper rifle will generally take less abuse than an assault rifle, when you need to displace in a hurry, you will drag your bipod across the ground and move with it extended, so if it's crap it will get bent.