https://www.airsoftsniperforum.com/ is where you will need to read and read more.
As @Druid799 said, it needs a lot of experience to build a good sniper rifle. (I was an airsoft techie some years ago for AEGs and I trusted a sniper rifle expert to build mine.)
A good sniper rifle is expensive. There's no way around that.
If you want to build your own, there are a few options:
1. VSR based. Cheap, completely VSR platform, and everything inside must go. The hardest to build.
2. A&K M24. Has lots of goodies already, hopup unit is not VSR compatible, I think it's very hard to find one now.
3. Striker AS02. Cylinder and piston straight to bin, £100 to get the proper ones. Then needs a suppressor and a suppressor adapter, that' an other £25ish.
4. Cyma CM.700. Also comes with 90 degree steel trigger, but plastic piston. Looks decent and is cheap to be a good base.
5. SSG. not that good as it is expensive. Still, good for an out of the box gun.
Chinese M40A3. I think ASG, maybe. VSR-ish internals. Not sure, but people like it. Fine if you don't want to invest much to get an OK gun.
Action Army AAC T10. The plastic piston, spring guide, and sear? is bad, and the fact that AA could have done it with their proper parts, but built it with cheap stuff is annoying. Still, a pretty VSR platform.
Any APS2 platform (most Well rifles), TM L96 and clones, cheap rifles (they are not really compatible with anything), any gun with the feeding ramp.