The P90 can be a workhorse if taken care of properly and it does have a V6 gearbox which is arguably one of the best designs. However like everyone else said there are problems with it in terms of real life play with ergonomics and the like. BTW if you do end up getting one down the road as a secondary backup incase your primary gun goes down UMP pouches will fit the P90 mags and vice versa I believe.
As far as first gun goes I definitely am a proponent of either an AR or an AK platform. However down the road you can get into unique stuff you like just have a solid primary setup and if in the case you have bad luck at a game and your primary setup goes down you can continue playing with your backup setup.
As far as other gear goes, you can do what you like and figure out what works and what doesn't for you yourself. Honestly if you have a tight budget you can get into airsoft for sub $600 with basic (quality) gear but with your budget you can jump in and get some more useful gear. Get rid of the helmet, faceguard, and those mP5 pouches and you should be able to shave some money off the budget.
Personally this is what I suggest;
Rifle $250+tax (feel free to use the highcap for now and store a second in your pocket, later down the road you can get a rig to carry midcaps and the like don't worry about ribbing about having "tactical maracas" it's all lighthearted anyways)
BDU: Olive Drab set (tunic, trousers, and bush cap) can be found at surplus stores for sub $70
Eyepro: $50 (get a paintball mask, the JT Flex8 or Proflex come highly recommended)
Boots: ~$150 get something you like and is comfortable. Doesn't have to be supertactical or anything. Use hiking boots in a dark/earth tone if you want. I recommend Merrell and Bates (parent company (Wolverine Footwear/Wolverine Sports) owns both and Saucony as well) I love their stuff. Definitely focus on good ankle support and comfort, your feet move you around all day and you stand on them and walk with them all day too, treat them well. Hell I even soak my feet in epsom salt after work and after games sometimes (don't have to believe all the newage bullcrap around it but it does help) and moisturize too.
Battery and charger: Can be found like $50 for a full set of like 2 lipos and a basic smart charger (doesn't have to be a crazy RC charger with a screen and a million options just a charger that you plug in and cuts off charging when it detects peak charge is good enough usually will have a light on it) EDIT: Talking about something like this btw, that's all you need to start with
http://hobbyking.com/hobbyking/store..._US_Plug_.html
Most of everything else is just accessories.