I would go lipo if your esc supports the low voltage cutoff. I think the esc that comes in those is a hobbywing and should have a low voltage cutoff.
The problem with a lipo is that if they are over discharged you will damage the cell. The cutoff will shut it down when the esc reaches that voltage keeping you from damaging you batteries.
I know when I was a kid it could be hard to stop when it slowed down... I remember running those old batteries until it just wouldn't move any more.
A good charger goes a long way with this hobby. Most chargers that will charge a lipo will also charge a nimh battery.
My recommendation would be to get something that will do lipos and if you want to hold off on actually using lipos for a while that's fine.
Eventually you will end up wanting to go to lipos and you won't have to buy another charger.
I think a 2100 to 2200 mah 2 cell lipo should fit. It might be worth taking it to a local hobby shop to double check.
As far as chargers go something like this might be a good idea.
Most local hobby shops should be able to get them or something like it. For 50 to 60.00 it will charge any battery you might need. It also plugs directly into the wall. Some chargers need a separate power supply and you probably don't need that extra confusion or expense.