Check out M Carter Brown forums,theres a few bolt and lever action rifles built there,mostly using old Nelson style pumps
If you want to start with a brand new marker look at the Kingman Hammer 7
around $140 new,mag fed for First strike or paintballs,I have one and love it!
I built a bolt action rifle out of a model 98. it wasnt easy and making it mag fed would be a bit harder too. I would suggest getting a hammer 7 and modify it to fit into a stock, wouldnt be that hard.