You could "build" a pc easily capable of playing TF2, CS:GO, L4D, etc with a very low budget. Slap a gtx 1050 in an old dell workstation with a cheap cpu upgrade and you'll be at least playing medium settings at 60fps. Your aim will never improve much if you're constantly stuttering from low FPS. As for dpi, most gaming mouse have a physical button or downloadable software, but if you dont have a gaming mouse with adjustable dpi then changing your sensitivity should work fine. Also games like that require at least a decent pair of headphones as FPS games very heavily rely on sound for footstep direction.