What you should go with really depends on what you intend to do with the buggy.

If you intend to just bash around or run on a track, the 9XL is a good start. It will give you decent speed and will give you more runtime before it overheats.

If running on a track is your primary concern, you really have to watch the weight that you put on the buggy. For racing, I try not to go over a pound (454 grams) on the battery. Too much battery can make your buggy handle like a boat.

Unless it's necessary for weight balance, I also try to avoid running packs in series. It's just a pain in the butt. I know that cost is a concern for you, but a $140 battery that destroys itself in the first month isn't as good a deal as a $200 battery that lasts for a year or more.

Running an 8XL in a buggy with a MM controller on a maxamps 4S2P 8000mAh battery, I was able to get the battery over 120*F.

My point is this...your brushless system is only going to be as strong as the battery that you run it on. After dumping literally thousands of dollars on batteries, I now only go for the best. Kokams, Flightpowers, and Neu.

You can get a Flightpower 4S 3700mAh 25C pack for around $170. I don't think that anyone can say that that isn't a good battery. I've never regretted getting the good stuff.