Tone007: yep, the 4000T has a coin-shaped lithium battery standard, and also has terminals for attaching an external battery....
LoBai: I actually never used the IDE on my 4000T, but I would assume it would be similar to that found on A1200's, etc. I don't think there are too many limitations beyond the usual >4GB issues for HDD's with some OS versions... ugh, too tired to post links ATM, just search for it.... CD drives shouldn't be a problem, assuming you have a suitable filesystem....
Other cool points about the 4000T are the 2 video slots, the room for drives,etc... (mentioned already) and I actually thought the headphone jack was handy.... Too bad my 4KT is pretty much dead ATM..... it's a great machine, When it Works...... :furious:
My 4000T is still not booting, it's in pieces, sitting in my living room.
As for upgrades for any Amiga, it's all about the accelerator.... that's the expensive bit......