Look at the free plans from finelineautomations.com they use 8020 for the frame and also sell kits of all the stages. They have their own linear bearings and rails but you can use skate bearings or just about any linear bearing/rail system you want.
I was going to use wood but after seeing the plans using 8020 I decided it was going to be much easier to work with. You do not have to use the fineline plans as is, but use them as a guide. The people at 8020 will cut the pieces you order to whatever lenght you want for a small fee.