Recommendations from a former a3k owner:
1 - Battery!
2 - Replace PALs with GALs - this will help with heat.
3 - Upgrade SCSI
4 - Upgrade Buster
5 - Ariadne for ethernet or equivalent.
6 - Thylacine USB if you can deal with 1.1 speeds (fine in my experience)
7 - CV64 with passthrough - perfect for this Amiga. Can't stress how perfect and awesome this card is for an a3000

Not the cv64/3d edition, just the cv64!
8 - ACARD scsi to ide and ide to cf, if you don't want a spinny disk - also great for heat
9 - WarpEngine 4040 modified to have only two SIMM slots parallel to the processor so it will fit in the case
10 - Some way to throw fans in there and keep everything cool!
Enjoy! It's a great computer for sure. If you don't want to modify a warpengine, a cyberstorm will do very well, and I did 99% of what I wanted with an a3640.