You can check out my build thread, my machine uses Drylin rails and some simple solid couplings and simple leadscrew endblocks. It works very well and is quite precise;
Diy_hobby_small_plastics_mill_router
As for your first build, especially if making PCBs is important I would recommend just making a smaller machine say 10" x 10" by 1.5" which will be great for PCBs and many small flat sheet jobs (acrylic, urethane etc). But with those dimenstions it will be easy to make rigid and accurate. On a small machine like that you can also use threaded rod for your leadscrews which is cheao and easy and will give you a lot of accuracy from the fine pitch.
When you make a larger machine later as funds allow the small PCB machine should be easy to sell although you will probably want to keep it for PCBs and use your larger machine for wood.