LiPos are much better than old NiMh batteries in every way. IMAX B6 chargers are really good too (I'm also wanting to buy one myself)
You want a 7.4V as if you're gonna use an 11.1 your gun will need a lot of upgrades.
Component Shop or Hobby King are good places to look for batteries, the former having a dedicated airsoft section too. Both can change the connectors for you as well if you've re-wired to Deans or something.
25C is the optimum discharge rate you're looking for IIRC. A rule to remember is that you get a shot a mAh (on semi) apparently. So the 2200 should be enough power for ~2000-2200 rounds.
The Raider is rear wired into the crane stock? I thought it was but I could be wrong depends on the exact model possibly. So rather than a normal battery it would take one of the split ones. My T4-18 has a crane stock and it fits the 2600mah 7.4V lipo Vapex batteries so I suspect it will fit http://www.componentshop.co.uk/7-4v-2600mah-25c-continuous-discharge-cranestock-lipo-battery.html as well. You can go for a smaller cheaper one if you like but those big batteries last more or less a whole day.
The basic really safe rule is 1 mah = 1 shot but most guns are much more efficient than that and its more like 800 mah = 1000 shots. I just go with 1 mah = 1 shot because its simple and easy and leaves plenty of room for counting errors here or there and allows you to work out when to change the battery off of basic magazine counts.
A Lipo 7.4 V at peak delivers 8.5V
A LiFe 9.9 V is at peak going to deliver 3.6V x 3 = 10.8V
A Lipo 11.1 V at peak delivers 12.75V
So the Life 9.9 basically splits the difference between the two lipos, its more than a 9.6 nimh however because of that peak voltage after charge. Lipos and life batteries are always listed on their nominal battery which is what you get for about 50% of its discharge. The life batteries have less energy density than a lipo, so they tend to be a little less shots for the same weight and space but its only about 14% less, they are still better in that regard than nimh (which honestly no one should buy they are rubbish).
I've made a battery box that sits just forward of the mag well, i used some box trunking lightly modified, sanded rough sprayed mat black and then rewired to be a 'front wired' rifle
I can use both lipo and nimh on it so I can switch between them during a game day