Well, couple things:
First, bolt action springs rifle are the single most reliable airsoft guns ever made, particularly the marui VSR-10 once upgraded. Work great in winter or summer, never break down (again, after its been upgraded)
Second, if you're even remotely thinking about getting headshots, you'll never be a good shot. Airsoft or real world, there's more to shooting than just killing someone outright. Especially in airsoft, a shot anywhere is typically considered a kill. And in more complex medic'd games, it's always more tactically beneficial to disable a person and force out a medic than to kill them. Headshots are always an 'only option' shot.
Third, performance. Forget about fps. Forget about shooting 300ft. Although I've made as many as the next marksman, long range shots were always just for storytelling. Because of my local field topography and foliage, my bread and butter was in the 20-120ft range. The two greatest advantages of my VSR-10 was it's accuracy and incredible silence. People will laugh at you for doing speedball or CQB with a bolt action, but it proved to be a gross advantage, being able to shoot through tiny cracks with incredible precision. Able to hold down 3 people with AEG's behind a barricade only 40ft away because you can shoot through any crack in it, and at the same time having people sprint out of spawn in fear of getting shot from the other side of the 300ft long speedball field.
Your two major advantages are going to be accuracy and silence. Assume AEG's can shoot just as far, but not as accurately, or quietly.
Being a marksman or sniper can be tough, doing that while also helping your team is even harder. Remember it's not just about you out there, your kills count for nothing if your team loses!
That being said, some people can do it, and some people just can't, but it's definitely worth trying, and if you do it long enough, you can learn a lot of skills that apply to any gun.