If it's relatively simple stuff, don't be afraid to dove into it yourself. I cracked my first gearbox with just a YouTube guide and a 5 quid tool set. I made mistakes sure, but now I know enough about how they work to diagnose problems and maintain it well
The only thing I'd consider paying someone else for us r-hopping, as I don't have a range handy to test it on to make tweaks.
Still, I know people who don't ha e the time to do it, and others who are too scared of messing something up who pay people to upgrade and fettle, but I just don't know who they use
My mates got a G&G SR-25 and it's a really nice gun. His has been upgraded to shoot just under 450 FPS and the range is excellent. The biggest thing to consider when getting a DMR is what FPS to have it setup at as site limits for DMRs vary from none(i.e. 350 FPS standard AEG rules) through to 450 - 500 FPS with sniper rifle MED rules.
If you are going to play a lot at sites with more restrictive rules then the less value you will see. For instance if you play at sites with a 400 FPS limit but still have the 30 metre medium engagement distance you may be better served getting a ~350 FPS AEG with a well setup hop up.