... You know filling in your location might help a lot.
Most of the gun designs are from pre 2000 so actually more than "3-5 years old".
You'll want a reinforced box just because CNC'ed boxes will probably be hard to find/expensive. Although I believe AirsoftParts has a CNC'ed box in stock for ~$200.
Mechbox versions are really just the shape of the mechbox. eg. M16 and M16 variants mechbox will always be V2, likewise a V3 will always be for AK and AK variants. There are improvements over each version. Such as the V6 being reported to rarely fail/break. Choosing one really only involves picking a gun you like (M16, MP5, AK). Then read the reviews about them. "Arnies Airsoft" is a good place to get reviews.
If you want a gun to play outdoors and indoors a PTW might be the one for you just because of the quickness of doing a cylinder change to switch up the speed, you'll be able to go from 500 FPS to 290 FPS in a snap (with the correct coloured cylinders).
Otherwise I believe ARES/STAR, and ICS make guns that have easy takedown to do spring swaps.
Also don't forget about Chinas Type 56.
As for accuracy, don't worry about that since all you need to do is put some aftermarket parts (tightbore (6.03), hopup rubber+unit, etc.) into it to make it as accurate as you want. But keep in mind that the dynamics are still akin to a musket or shotgun rather than a rifle. If you're using this to shoot pop cans then might as well get a pellet gun.