Before dropping serious cash on upgrade parts, I strongly recommend you read
this excellent guide to the DIY aspect of building a marksman's rifle. You'll be able to get the best performance out of the stock parts you're keeping, learn a bit more on the theory, and in the process you'll get VERY comfortable taking apart your gearbox and diagnosing problems (there will probably be tons of them). Bonus: most of this will not cost you very much if you've got the tools.
Before embarking on this project it's important to get a chronograph, it will be impossible to judge your FPS consistency without one.
Lastly, once you've gotten your stock/nearly stock gun to the point you don't think you can improve it any more without upgrade parts, make sure the brands you're buying are any good with
this handy guide.
Good luck, hope you don't run out of money/patience before you're finished. This is a big project and it'll be pretty tiresome at times.