I just got into the sport as well... my biggest thing I found when buying the gun is go hold a few. See what you like in your hand, what is the easiest to snap shoot with, what kind of sites do you find the most usefull. I let my GF hold my AK before we bought her p90, she could not even reach the front stock on my ak lol had to put her hand by the mag. So yah old a few first.
I got a TM AK47 and I love it save a few things. I picked it up for around $400 with battery and Fuzzy logic charger, came with a low cap and high cap mag. Only had about 1500 rounds through it. So the deals are out there, don't rush the purchase and you can find a smoking deal.
I am personaly cautiouse of upgraded second hand stuff. Its like buying a Car that has been turboed, you do not know if the owner has built the engine up properly, and even if he has how many miles are REALLY on the parts. Get someone to look it over for you before you purchase it or you might end up with a headach like my gun doctor thread was... in the end it was just me worrying but still... could turned out to be much worse.