Here's my advice for mid caps. If you buy em, you NEED the mag pouches AND a dump pouch. Otherwise mag changes under fire SUCK. With those two things, they're fun!
For the gun, the first thing I would do is ditch the 8.4V battery that comes with it. Either with a Li-Po or a 9.6V Ni-Cd. Also, if you buy a Li-Po, search ebay first or hobbyking.com or something. They're super overpriced at any airsoft specific place.
I would advice something cheap at first if you're looking for Molle, not a full plate carrier. I wear a Fighting Load Carrier (FLC). They're cheap (~$15) and have quite a bit of Molle real-estate.
Camelbacks are a necessity for sure, especially once it starts heating up. I have an off brand bladder that I've had for probably 7 years now and it still doesn't leak.
One thing I decided I NEEDED after a couple of OPs was a set of knee pads. I don't know how I ever lived without them nowadays. I bought mine used.
I'm kind of a cheap bastard in case you haven't noticed. It really makes you prioritize though, when you want to spend the least amount of money possible....