I don't know of anyone with 223 kits right now, but ORF as mentioned above and maybe K-VAR are about it for 74 kits now. If you want a 7.62x39 kit, then Copes Distributing and/or DPH Arms are both good. If it's your first build, I'd recommend starting with a Romanian "G" kit from one of them for your first one... they're around $120 each. That way you can practice on a cheaper parts set first without worrying about messing up a more expensive kit.
As for what you'll need (not counting build tools)...
1. Parts kit - A "matching numbers" set is most desirable. Starting with a matching numbers bolt, front trunnion, and barrel will avoid most headspace problems.
2. A receiver to build on. Flats are cheapest, but most difficult. They have to be bent on a bending jig, heat treated, bolt rails attached, and generally require a fair amount of final fitting and trimming. The easiest and most recommended method for the first time builder is to buy a 100% ready receiver to build on. NoDak Spud makes the best AK recievers... their NDS-3 is your best bang for the buck in a 7.62 build. They also have various 5.45 receivers if you want to do a '74 build instead.
3. Fasteners of some sort to assemble everything. Rivets are generally preferred, but some people choose screw builds. If you choose to rivet yours together, Curtis at AK-builder.com has the best rivets available. They are easiest to work with, are presized to correct dimensions, and look better once installed than any others I've worked with. I've never built a screw build, so I don't have any recommendatons on sources for proper screws.
4. 922r compliance parts to keep it legal. There's plenty of info out there on 922r and building AK's, but the short version is that you need at least 6 US made parts on it to be legal. The easiest way to achive this is to buy a USA made fire control group (3 US parts), a US made pistol grip, a US made muzzle break, and your US made receiver brings the total count up to 6 US pieces. Most any of the links mentioned above will also have the 922r parts you will need.
Not absolutely necessary, but extremely useful, is a fire control group retainer "L" spring or FCG retainer plate... both are much easier than trying to cut and fit the original "shepard's crook" spring to hold in your FCG pins. AK-builder.com has these as well.