Removal of the old battery is definatly a good idea, I did years ago but never botherd to put another back in and my 4000 still runs great just the clock and date isent set but I dont care.
OS 3.9 also good, look at getting a good graphics card unless you want to install a Video Toaster card for some real fun :-D
and get a backup 68040 CPU board as they do fail.
and lastly put a fan in or outside of the case on the left side to keep it cool.
