Don't get fooled by people who tell you to get every upgrade available. We call those kind of rifles Lego builds and the shooter usually couldn't hit anything if their Lego gun was even tuned in the first place.
Take the time to read and learn about your gun and what is really required. Most of the time a good barrel, hop rubber, piston and at least sears (again, it depends on what you start with) gets your platform reliable. As already mentioned, use quality heavy weight bbs suitable for your fps. A .28 is a bare minimum. A good scope is a must. I've been using a bolt action rifle since I started over 7 years ago and never once used a bipod so don't let your purchase decision hang on that.
Sniper rifles take a lot of time and care to tune. Consistency is most important. Go over every piece of the gun and shim or trim out any play that you can. Make sure you clean it often, use the right lubrication.
I rarely need to switch to my sidearm. Get a basic reliable propane powered gbb and an extra mag or two. If you want to be sneaky a nbb and silencer can't be beat.