Your batteries will definitely be hot after running for 30-40 minutes! Most races are only 5 or 6 minutes long.
Simple Rule: If you install a faster(lower turn) motor you should also install a smaller pinion gear so you do not overheat your motor/esc. Keep your gearing around 10:1 to 11:1.
Sample: 78T spur gear/18T pinion gear x 2.5 internal ratio= 10.83 Final Drive
Refer to your instruction manual for internal drive ratio.
Adjust your gearing to the track, you may want more punch and less top speed or vice versa.
Monitor the temp of your motor/esc, the motor shouldn't run hotter than 160'.
You can definitely run NIMH with a brushless system. Some guys prefer running the NIMH because they weigh more than a LIPO and the car will usually handle better with more weight depending on set-up. If you run a LIPO you can always add lead weight to the chassis wherever you need it to improve handling.