Nice plan. 75% of the fun is finding the parts you want. My mag started off as a minimag I bought in 2001. All I have left is the stock trigger frame and I don't use it. I've spent enough money on parts to fund a small 3rd world country but I really don't care. I've got the only paintball gun i'll ever need and definitley the only paintball gun I'd never sell.
Good starting point would be ULE body.
Post pics when you get everything done.