If you buy a old tractor you want the least amount of electronics possible. Mice love wires . Moisture kills electronics computers . KISS !! keep it simple stupid.:cool2::drink:
I have a 4720 and yes the amount of electronics is scary. The front engine cover also leaks antifreeze a little but not terrible. I know if it goes down it will be $$$ to repair. It has plenty of power for sure.
I bought it used and it has about 980 hours on it now. No issues to date in the almost 4 years I've had it. It is stored indoors and I don't have any evidence of mice in the 14 years we've owned the house and garage other than in the wood pile.
Having had a 4120, either will have enough power. Depending upon your use, you'll run out of traction before running out of power. If you purchase, go ahead and fully service it. Then take out the battery, clean the box and repaint. Happy hunting!