hayashi: If you can find/afford it a hard drive is IMHO a near neccesity for any Amiga. A second floppy can be handy, if for example you have to copy disks regularly, or make disks from .ADF's and/or .DMS's regularly, but this should be
very low on your list. I have just one external Amiga floppy drive that 'migrates' to whatever Amiga can use it... :-D
An acellerator would be nice, e.g. for any serious stuff, but be aware A500 accels. can be quite expensive....
Hmm, OS 3.1 vs. 2.1

Well, from personal experience, both versions ran about the same speed, but I can't even remember what it 'felt' like with less than an '030 @ 50MHz.... Game compatability maybe tricky with 3.1, more reason to get / register WHDLoad when you get that HDD !
DPaint5: Not too easy to find, atm. Second, it's a memory and processor hog. IMO, it can do very smooth animation, has AREXX, and actually displays all the fonts on your system, unlike previous versions... TBH, I still use DPaint IV and/or Personal Paint 7.1 (Given away on a cover CD) for most work...