Highcaps (the large capacity ones you have to wind) are frowned upon in many if not most serious airsoft groups for a variety of reasons. So you're likely to want to get some real/low/mid-caps at some point. Whether you get real/low/mid depends on your play style and how realistic you want to be. Personally I got into airsoft primarily for the realism so I'm realcap all the way.
As to Vest/chest rig, that depends alot on personal preference, play style and partially on your size. Big guys can full-on plate carriers easier then little guys can. For the most part, a chest rig should suit your needs better then a full vest or plate carrier. The full rig is great when you're doing a week long patrol in the middle of the sandbox, but for half hour skirmishes less then 50 ft from your ruck you really don't need to be carrying all that much. For molle versus fixed rig, there are pros and cons to both. Molle is generally bulkier, and considerably more expensive, but is configurable. Fixed rigs are more streamlined and considerably cheaper, but as the name implies the pouches are fixed and can't be changed. I personally knew exactly what I wanted in a chest rig and knew I was unlikely to change when I went shopping, so I purchased a fixed chest rig and haven't had a single regret.
As to extra gear: If you're getting a sidearm you're going to want a dropleg holster. This is going to depend a lot on the pistol you get but considerations most people think about are wobble when moving, solid retention and quick draw. I personally care much more about the first two then the last, so I got one that is extremely solid on those two at the expense of the last. Price tag is another consideration. A simple nylon holster can be acquired as cheaply as $30, or a full-on SERPA tactical rig for in the area of $170 and everything in between.
A few other pieces of gear you might want to consider:
good eye protection (ESS goggles are great)
good footwear
a red dot sight
duty belt
sling (one point or three point)
a drop pouch for spent mags
keepads (Alta makes great ones)
shooting gloves
camo bdu
some sort of hydration (simple belt canteen or camelbak)
~8-10 mags for your rifle
maybe an extra mag or more for your pistol (depends on what you use it for)
gas adapter (so you can use basic propane cylinders for your pistol mags)
spare battery
good charger (universal smart charger is a good affordable choice)
gun case
a decent ruck and/or seabag to carry all your gear
The total for that list above is about $900, and that's getting a considerable amount of it second hand or cheaper versions. It could easily be as much as $200 more. Add the ~$350 you paid for your rifle and the ~$250 you'll pay for your pistol and you're starting to talk some serious money. When I step on the field, I'm wearing or carrying about $2500 in gear. That's not even counting what's left in my ruck which is about another $800.
Airsoft is not a cheap sport to say the least. It's generally accepted that the absolute minimum you need to get into it is $500, and it's usually more like $1000-$1500 if you go with quality gear. I've seen a few people do it for less then $500, but it takes a fair bit of time, effort and jury-rigging. And of course quality tends to be a problem at that level.
My Classic Army M15A4 Tactical Carbine without external addons but upgraded internally
(total cost = ~$600):
My baby, CA M15A4 Tactical Carbine with m203 grenade launcher, red dot sight and taclight
(total cost = ~$1000):
Me in my full primary kit
(total cost including the guns and a few things you can't see in the pic due to angle = ~$2500):
