If you don't plan on using your Amiga for Internet use, go with Workbench 3.1 . It's good and cheap and you can add patches from one of the distros from the emulation scene. It also comes on Amiga formatted floppy-disks so you won't need a CD-ROM drive to install it.
Otherwise go with AmigaOS 3.9 and both Boing Bag expansions from
here. It's a little heavier than the earlier versions but an '040 should be able to handle it.
