[AoiShikaku]

I'd agree with the WE GBBR.
I've got a WE KAC PTW Open Bolt and have upgraded the living shit out of it. There are so many things available for the unit and it all depends on what you really want. Another team mate of mine runs a WE M4 and his runs pretty damn awesome.

I did have leaks in a few of my mags, but on the bonus side there is a guide to having them all sealed up done by Zong (I played it safe and resealed all my mags).
For the maintenance guide that would be from Shelled Pants.
Both guys are really cool and knowledgeable about guns when I met them.. after talking with them briefly I bought a gas gun later that week.

Also.. if you ever break something by accident.. great part is that everything is available.. some at a really cheap price... like $3-$5 for a valve.

If something goes wrong in my gun.. can usually be taken care of in under 1 min on the field. =)