Romanian AKs seem to be going for around $350 these days in most places. Century Arms and J&G Sales are good places to start looking. Samco has Yugo SKSes for $180-$250 if you want to go that route.
I think that an sks would be your best bet, the best being the YUGO m59/66. They're not too expensive (mint condition for < $300), they are made very well (like most SKS's), and they have a grenade launcher. While you can't buy grenades, you could probably still find a use for it if you ever needed to.
I say this with no disrespect intended. Please consider that the use of politically-charged terms such as 'assault rifle' can feed the flames of the hysterical and uninformed who oppose our rights to possess firearms.
After all, this was considered an 'assault rifle' at one point:
Out of those you list I would go with the SKS or either of the SAIGA carbines.
I'm a big SKS fan and a little biased there, but love the AKs too and think Saiga are really nice quality. Since both are in the same approximate price range you could go with either. Ammo for the .223 is pricy right now so i'd go with 7.62x39.
Buy an AR-15 lower & build on it as you can afford it. Buying cheap stuff gets you nowhere. Think about the ammo you could possibly blow $$$ on while playing with a cheap gun as parts you could buy towards a decent rifle that you can always add to.
I am with a lot of others here and throw a vote toward the SKS. I have one in a polymer stock and added a 40 round mag. Now, I got mine for $75 so at the gun show this weekend I nearly threw up seeing SKSs for $250 and up.
Save your $$ for an AR platform.
You can find a good Ar15 for $700-$800 or build one of your own for less. Then buy ammo in bulk and learn to reload.
Reloading will save you up to 50%.
You will not be disapppointed in an AR15 and AR does not mean "assault rifle".
You can buy an AK47 for as little as about $400. You can shoot steel case in it all day and if breaks you have lost nothing.