Hey, sure thing. If you want to go with the HPI car, you will need all that you listed above (the four points). The difference between a four cell and six cell is the voltage. The four cell will have 4.8V whereas the six cell will have 7.2V. The extra cells will provide more tope speed, perhaps better acc, and battery life should be the same. Motor life should increase though, because you aren't running as much voltage through the motor and therefore not wearing down the brushes as fast as if you were running on six cells (assuming you are running on brushed motors).
As far as batt/motor combinations, I would get a 6 cell pack for ease of finding one. This one should give you plenty of runtime:
http://www3.towerhobbies.com/cgi-bin...&I=LXSNE1&P=SM. The mabuchi motor will work fine, and if you want to get that one, you will have quite a bit of runtime with the batteries mentioned above. If you want more specific examples of what I would get if I was doing this for less than 300, just let me know

. Keep the questions coming, I'm glad (and I hope) this helps.
Traxxas Rustler, AE RC10T, Kyosho Mini-Z F1, Tamiya M03L, Tamiya TT-01 Custom Buggy (in the works)