As an owner of 2 A3000's.. a 16mhz and a 25mhz,
my advice is (unless you have extra-ordinary good
luck and lots of money).
First don't worry about the origonal chips, they
all work fine.(I've owned my A3k-25 since new and
never found the need to upgrade any chips, (SCSI etc.))
I do have a WarpEngine in the A3k-25 (runs great)
In the A3k-16 I installed an A3640 accelerator (much
easier to find than a WarpEngine) and 16 megs fast ram.
As for a harddrive I've found the Quantum drives very
Amiga friendly (the easiest to setup). I'd go for a 3.2
gig 50 pin drive. I'd also get a set of 3.1 roms then you
can run OS 3.9 (although OS 3.1 is nice)
And if you are lucky enough to find one, a CyberVision
64 GFX card is is a great addition.
Oh, and as far as the coin battery is concerned ... the
3 volt coin battery probably only works in less than half
of the Amigas its tried in (I'll probably get an arguement
here ;-) ) Also I've been using my A3k-25 for the last 3
or so years without a battery with no ill efects.
Other than that.. I'd say use the machine for awhile..
before deciding what else you want to do.