Good for you on taking the plunge. The great thing about the hobby is you can move along at the pace you choose.

I don't know much about onroad, but the 8000 mAh battery is probably overkill. The lighter 5200 pack will likely be plenty.

Charge your battery in balance mode, and always use a balance cable. If something has gone wrong with a cell, the charger will catch it immediately in that configuration.

You'll have to bust out the manual for your ESC and make sure you have the lipo cutoff enabled and again, I'd bump it up to 3.4 volts to start with. You could go down to 3.2 and still be okay.

The lipo may be lighter than your nimh pack, so check your ride height.

Have fun, and report back on your lap times!