You only need 50 to repair diamond, I highly suggest starting out with cobblestone for a level or two unless you have a lot of iron.
The BEST thing for building repair skill is to use gold, if you have an abundance, because it decays fast and it gives mega repair skill exp.
You can see the info here:
http://mcmmo.wikia.com/wiki/Repair
I have ~600 repair, and on a good roll I can repair a diamond breastplate from ~25% to 100% with one diamond now