1. Get a quality mask, v-force profiler, JT flex 7/8, Dye i4, the new empire is nice also but make sure to get a dual pane lens if you go with the profiler and if you have the extra money get a fan kit for it. You may only need to use the fans ever so often but from my experience in long scenario games the fans are invaluable and will keep you in the game even if wearing glasses.
2. I used to go for the milsim style guns as did most of my team but now after using a variety of tippmans, TM 7/15's and other milsimish guns and then switching to a space gun style its a world of difference. Speedball guns are lighter, are easier to maintain, and you can get a good quality one for a few hundred. Some speedball guns do allow for adding a sling adapter and stock. The AGD tac one would be a good pick as it has some pictinny rails to add stuff but keeps the function of a speedball marker, and its mech so no need to worry about batteries for the long games.
3. Go with HPA only if thats an option. You will get better consistency and more shots per fill. A 68/4500 or 45/4500 are good options and if you want a lighter set up run a remote line.
4. Valken makes a good vest called the echo. It holds 9 pods, has a space for adding a tank pouch, has some molle on the front straps for a radio and whatever and has a good waist attachemnt that provides good back support.
5. For scenario/milsim games look into a radio. I use a midland with a press to talk and agent style ear bud (clear coiled, forget the actual name).
6. If you end up leaning towards pump (which is a great idea IMHO) id go with a ccm, empire sniper or phantom. All will do the trick but the ccm is by far the best for pump stroke. You can't do semi with it but most can get 6-7 bps with one. Bonuses of a pump are no batteries, dont 'need' a electro hopper, they are light and are easy to fix.
7. Ive never used a q-loader but when i brought it up to my team they strongly urged me not to. From what ive heard they are a pain to load and can have issues with working properly. If your going to do long scenario games a 200 round hopper will give you less reloads and pods are way easier to load up with. I know getting rid of the hopper sticking up on your gun my be a goal of yours but you need to find out of its worth the extra hassel of the q-loader. If having a low profile gun is important look into magfed guns like milsig or tiberius. Your rate of fire and ammo will be greatly reduced but it will come down to your play style.