It really depends what you're looking for. If I had enough money, I'd just get something I thought looked cool, throw it at a gun doc and let him deal with it. Unfortunately I don't, so I try to be as cheap as possible. So for me, airsoft and gun doctoring is one and the same. Maintenance and repair is inevitable so don't cheap out on guns! The cheaper brands will cost you the same or more than a nicer brand in the long run.
Because I do everything myself, I want it to be as hassle free as possible. That why I would only recommend two guns: TM p90 or G&G fn2000. Open the back, unscrew two screws and the mech box pops out! Shimming a version 6 is really easy because of the motor cage, you can easily get the motor alignment perfect! P90 is compact and light even with a silencer. Only downside is the magazine! They're hard to find and typically not as cheap as m4 mags. Also getting pouches for them at a reasonable price is a bitch. I've never used an FN2000 so I technically shouldn't be recommending it but I've read great things about it and it's easy to maintain! Fn2000 takes m4 mags so that's a big plus!
Regardless of what you choose, I highly recommend staying with a gun that has TM compatible parts. When something breaks down, you'll have a much easier time finding something within your budget!