I think a 15x2 will be plenty fast enough. The only time I had problems with brushed motors was racing indoors where there was a large jump straight after a hairpin. Brushless motors just have more torque and accelerate harder. As for cells, Demon Power Products do 6-cell packs.
http://demonpowerproducts.co.uk/index.php?cPath=46_47
You will need a charger that is made for NiMH to use them, NiCad chargers definition of 'charged' is different than for a NiMH charger. Most modern chargers will charge NiMH and lipo.
As for motors DMS has some decent buget modified in the sale section
http://dms-racing.com/
Older buggies just weren't designed for the sort of torque that you get with brushless, and brushless will just eat diff and outdrives.
If you are tempted to get a brushed esc, then I'd make sure it has an inbuilt lipo cut-off for upgradability.
One other thing is that I have seen a 27t motor go up in smoke when running Lipo as lipo can provide much more current under load than NiCad and sometimes its just too much!
Hope this helps
Ben