Well, you will need a barrel, buttstock and forend. If you need/want to scope it, add a mount, rings and scope. If you are not scoping it, make sure that your barrel has sights.
I'd look at Ed's Contenders site (
http://www.edstc.com/index.html), he has everything you'll need, including used barrels when available. Also, everything you'll need is available in the Contender Classified forum sooner or later.
I've got 2 Contender Carbines now, one using my pistol frame, and the other a G2 rifle frame. I have .17 HMR, .22 Hornet and .30-30 barrels. Both have walnut stocks, but I have an Encore with the Rynite stocks. I prefer the looks of the walnut over the plastic, but both work well. Remember when ordering: G2 frame buttstocks and earlier frame buttstocks are not interchangable.
These prices are from Ed's:
23" .223 new barrel - $225
Contender Carbine Walnut Stock and Forend - $140
Contender Carbine Rynite Stock and Forend - $79
Leupold Contender base & rings - $38
Just add the scope of your choice, and go!