I love a good AK rifle. It is my preferred battle rifle since they are so incredibly reliable, reasonably light-weight, well built models are quite accurate, and the ammo (7.62x39, 5.45) is relatively cheap. These are certain things that will help you learn what you need to know about AKs. You can Google these items and learn much more about them by reading all the web pages that pop up.
1. milled vs forged receivers
2. chrome lined vs non-chromed bore (chrome is much, much better)
3. single stack vs double stack
4. varying quality of US made parts for 922r compliance
5. varying quality of different folding stocks
6. different calibers (7.62x38, 5.45, .223)
7. different mags types (steel, polymer, waffles, slab side, bakelite, 20-40 rounders, etc)
8. Methods for scoping AKs, if that is something you want to do. I don't, personally.
9. last but certainly not least, the US parts needed for forged and stamped receivers to be 922r compliant.
Here would be a great one to start with. I can't believe that they are up so high now. A few years ago you could have gotten this SAR for $300-350. But this is an excellent rifle to start with.
http://www.thehighroad.org/showthread.php?t=272601
Just to give you an idea of what can be had, I've shown this off more than once, but here are my AKs.
http://www.jasonphillips.org/offsite/AKbros.jpg
The SLR-95 on the left hasn't been imported into the US in quite a while. It was brought in by Arsenal and has a Bulgarian milled receiver with a chrome lined bore. It came with a thumbhole stock originally. I put a K-Var stock set on it and I bought 4 sets of FSE mag followers and floor plates to get the required number of US Parts that I need (5). That way I didn't have to replace the trigger assembly, as the one that comes with the SLR-95 is very nice with a very clean trigger pull and break. I then replaced the K-VAR stock with an Ace folder and I replaced the stock sight with a Mojo peep sight. I love this thing. I bought it about 4 years ago from WildAlaska for $425 and the other costs were K-Var stock set: $100, Ace Stock: $150, Mojo sight: $45, FSE parts: $20. So I have about $730 into the rifle and it's worth every penny. Acceptably accurate for an AK (about 4-5 MOA), excellent fit and finish, excellent parts quality, and of course utter reliability.
The VEPR on the right is a typical VEPRk in 7.62x39. These were based on the RPK with a heavier forged receiver with walls that are 1.5 times the thickness of a normal forged receiver. It has a chrome lined barrel as well. Many of the parts are not standard AK parts though, so if one breaks, I will probably need to get it milled as parts availability is really up in the air now. But I have never heard of anyone having this problem, ever. The rifle hasn't been imported for a while now. Atlantic Arms was trying to start importing them again but I'm not sure if they have been successful yet. These rifles have a reputation for being very accurate for an AK. I can get 4 MOA consistently. The trigger has very smooth travel and breaks very cleanly. I picked this one up for about $400 two years ago with a PSOP scope (that I sold).
I don't scope my AKs. I shoot only slightly worse without a scope at 100 yards (like 1-2 more MOA) so I don't see the need for it. I would put a scope on a nice bolt rifle, I have never seen the benefit of putting scopes on battlerifles.