Marui VSR-10s are really good platforms for making a rifle.
Don't buy knockoff sniper rifles, bad platform = bad rifle
And for starters; tightbore, hop rubber, and 380fps spring. That will get you some pretty decent range and accuracy for around $160
Then once your trigger fails or you want better consistency, get a PDI cylinder set, V-trigger, and 400-440fps spring.
Most field limits are 470-500 (I think??), anyway you REALLY don't need to be above 440fps on .20s with a sniper rifle.
Above 440 your going to need something heavier than a .30g BB, and it gets pretty pricey....
And yes, ironic; an exclusive LMG gunner complaining about the cost of ammo LOL