It'll run you roughly $340 for the base RT with a black X-valve and no barrel, at the AGD website. That comes with the stainless steel body, and I'd say just swap that out with the new mag body. From there, it'd run you $90 extra for either the Intelliframe or the Y-Grip which are both double-triggered, and $59 to include an extra cocker-threaded two-piece J&J barrel.
That'd be the quickest way to get all the parts together, including an X-Valve.
As for the questions:
1) No, AGD markers are only compatable with AGD-style grips. There are some after-market AGD trigger upgrades, like the hyper frame, but 'cocker grips are not inter-changable.
2) The rail mostly just connects the body to the grip, and has a place to screw on the fore-grip (or battery housing, if you're going electro). It's not integral, but you can't assemble a mag without one. So yes, you will need a rail. There are plenty of after-market rails that are milled for lightness, also.
3) For cheap? Not really likely, but you might look for an old RT Pro (not just RT, has to be RT Pro) valve with a level 10 upgrade. Those might be a bit lower in price since everyone's going X-Valve lately. As long as it has a level 10 on it, the RT Pro valves aren't much diffirent. Mostly, just that the X-valves are alluminum, and the RT Pro valves are stainless steel, and a bit heavier. Again, make sure it has a level 10, or you might as well put down the big bucks for a new retail X-Valve. Again, i highly reccomend buying an RT Custom from their site, and swapping out the body.
4) All you need?
http://www.automags.org/forums/
Everything you need is there. If you don't see it, use the search option.