As already mentioned, getting 3.1 KS ROMs is a definate. (It's needed for 3.9). 3.9 comes with a TCP/IP stack as well etc so if you want to get on the internet with it you have the software needed.
Also, the motherboard can take upto 16Megs fast, you might want to see if you can pick up some second hand cheap memory.
You also have a 31kHz monitor port on an A3000 so you can use cheap PC monitors with native modes and also if you get a GFX card it will work fine with one of them as well.
If you don't want to buy an IDE controller, get some old SCSI I/II devices like CD-ROM drives/HDs (Ebay is good).
Accellerators and more RAM are a pain and can be expensive though. You just need to keep an eye out for good deals.
A3000 = ECS, A1200 = AGA, so if you want to play AGA games etc, the 3000 is no good.
