Whats your price range? If I were you I'd look at getting some plans. Plans make things so much easier and faster! See the Sticky message at the top of the forum for some plans.
As far as motors, I like big motors but that makes things so much harder. I use big honking nema 42 (really big), 1500 ozin peak, that have to be geared down. But I have seen people use small 140 ozin motors that are just as good for their application. Are you planning on using a big wood router or something like a Dremal? It makes a difference. If your going to be cutting metal you probably need something a little bigger than 300 ozin (guesing).
As far as two extra axis you should look at the
Rainea CNC Machine. Thats one way to do it.
Also, depending on your price range, you may just want to buy a machine. Ebay always has some interesting machines for auction, I'd check Ebay first. But you might also try the
Liberty CNC Machines. That looks about the size your talking about. Granted, you can build one yourself for much cheaper, it just depends on what your looking for. Also check out
Cheap CNC. They have kits.
From your questions I think my best advice is look for some plans. You will save yourself a lot of time and trouble. Hope some of this rambling helps!