Are you in close proximity to 1 of AEX's stores? I'd highly suggest you go visit and test fire whatever you want. I'm assuming you are lookign for an AEG (Automatic Electric Gun).
In Tokyo Marui's line of AEG's, they are all reliable and affordable and fall within your $300 budget inc beginner package. I'd suggest you look at the lines with the most after market upgrades. M4/M16's, AK's, G3's, Mp5s, P90's, M14's. The TM Uzi, Famas, Sigs and Thompsons are not easily upgradeable and have limited 3rd party upgrades.
Either way you go, stock TM AEG's will all fire about the same. It's mostly about looks. There are differences like between gearbox versions, but that's a LONG explanation. So you want to look for something that you can affordably upgrade at a later date.
CA, G&G, G&P, Star, VFC, Top are also good AEG's but they all need some tweaking of some sort. Luckily AEX backs everything they sell (Sometimes WAY outta warranty).
Don't be afraid when you narrow your search down, to ask AEX directly of what they think. They'll give you the lowdown on a reliable AEG based on what they are seeing come back as returns.