Depends what you are going to do with it. For me I tend to want an Ethernet card and 32MB of RAM plus external removable media (MO, tape drive, Jazz whatever). For OS 7.1 I never seem to need the 32MB of RAM I have so 128MB is a bit overkill (and I do have extra sets of 4x16MB 30 pin SIMMs on the shelf). SE/30's are not too slow for most software you would want to run on a low res B&W screen. If you have $100's burning a hole in your pocket then you could get a 68040 + greyscale adapter and whatnot.

The original SE is a bit slow so an accelerator is useful for those systems, but its a tradeoff between Ethernet and a CPU upgrade. One of my SE's has a 68030 upgrade (basically turned it into an SE/30) and external SCSI Ethernet, the other just has Ethernet.

Ebay seems to be the place for exotic upgrades, or you can ask around here in the wanted section.