[RC-Monster Mike]

Brushed isn't significantly less expensive than brushless these days - though it offers less performance in most cases. Likewise, Lipo batteries may be even cheaper than nimh per mah these days(unless you already have a pile of nimh you plan to use). 45minutes to an hour runtime will dictate low performance, or a LOT of battery regardless of the motor system used, too. Low speed tooling around can get you pretty good runtimes, though.
As far as a 540 or 550 motor - they generally use the same bolt pattern(1 inch or 25mm), so either one would install onto a typical motor mount. the 550 is a little larger, so recommended for heavier vehicles(more torque generally).