Yeah, sounds likeyou got a deal to me. Get the 3.8, you'll be happy. Heck get a bunch of different motors. My 3.2 is almost faster than my 3.8 in smaller areas, but I still need to get the gear set. I've heard lots of great stuff about the R-spec around here.
NiMH batteries do have a longer run time, and are slightly punchier, but are not a total must. Read this
Peak Charg'd post that TinyRC made, and this one
Peak charge with your volt meter that I made. I'm getting 20+ minute runtimes from the stock battery and a 3.8 motor.
My Microsizer Sport didn't have the crip cap on the controller, but it did have the one on the car. I don't know if the standard Microsizer is the same. Check out this
controller mod from the Quest for Madness website. It'll show you where the crip cap is. This
Car Rx mod will show you where the crip cap is on the car. If your not too handy with the soldering iron I wouldn't reccomend the car Rx mod, as I found the result to minimal in both the Microsizer, and Bit, but the Bit also had the crip cap in the controller, and the combined results made a drastic difference in that car. From a stuttery 5 feet, to 20 ft. indoors. If you have the one on your controller then just snip it off, it's not problem.
Dave