As i posted on the previous page, if your bashing i would go for the GP cells as they are easier to look after, make sure your charger supports them as well, and isnt just a wind up timer job which could under/over charge the pack and damage them.
Now most standard batteries come with a
tamiya style plug which is pretty inefficient and has quite a lot of resistance, i would recommend replacing them with
deans plugs which should only cost a few quid and are much better.
As for speed controls, the lower the turn limit of the motor you use, ie a 10T limit, if you use a 19T motor then it should not get as hot as if you were using a 10T motor, but be aware not to run the car for excessive amounts of time or multiple battery packs back to back as this will place high load on the speed control and also the motor.
After running, as a guide, you should be able to hold your thumb on the motor for 3 seconds before it gets too hot and it shouldnt really smell at all.
When looking at motors the first number ie a 19 x 2 is the number of turns the wire has around the stacks of the armature (the bit inside the can which spins, the stacks are green), the lower the more RPM which is produced. The second figure is the number of strands that this wire has, generally 1 or 2 strands are used for on road, but they wind up to 6 i believe, with them being refered to as singles, doubles, triples, quads, quints and so on. A lower number of strands gives more "punch" so a higher number of strands gives a smoother feel and is easier to control and is slightly more efficient.
As a guide, if your dropping the number of turns on your motor, you also want to drop a few teeth on the pinion gear as well, otherwise you will be overgearing the motor, performance will suffer as well as the motors lifespan (as above i gear for the motor to be warm/hot after 5 minutes of racing).
When you come to installing your motor, i would recommend soldering the wires from your speed control directly to the endbell of the motor for efficiency reasons, although you could install deans or corally connectors (these are small gold bullet style plugs which are very efficient, but you could mix up the polarity, no risk with the deans) to the endbell to allow you to swap motors quickly if you wanted to. When you wire up your motor, ensure your polarity is correct, the endbell should be marked, with a + on
one side (just below where it says epic).
Ball racing your car is a good first upgrade, it greatly increases the efficiency of the drivetrain, which if you are looking to upgrade your motor is the upgrade to do first, a hopped up motor would give a lot more stress on the drivetrain, and the ballraces will allow it to run a lot more smoothly and helps keep temperatures down on all your equipment.
If your looking at something else to upgrade, quicker/stronger servos are a good idea, stock servos are fairly slow and dont have much holding power, ideally you want one which will turn no matter how quick your vehicle is traveling and can hold the rotation under load. These can run pretty expensive up to £100ish but there are lots of good ones to be had around the 40-50 mark, as well as ebay or looking at importing them also.
Enjoy!