While an 8100 is a first gen high end Power Macintosh, probably don't get one of these as they aren't easy to work on. It uses the same tower design as the Quadra 800, which is hard to upgrade the RAM or remove the battery (if it haven't exploded yet). The potential of broken plastics if you open it up or take parts out the wrong way is a concern.
If you ware looking for a Mac that can run Mac OS 7 (for compatibility with older games), 6100 or 7100 if you want a Power PC 601 or the 7300/7500/7600/8600/9600 are better bets.
I don't recommend a Performa 6200/6300 since those are really slow and not that great Macs, they are really slow, but you can use cheap CF cards, but there are no G3 upgrades for them, except for those rare cache card PowerPC G3 accelerator cards. 6360/6400/6500 are okay, but limited G3 accelerator options, and can only go up to 128 MB of RAM.
If you don't care about Mac OS 7 or need lower color modes, a G3 or G4 Power Mac towers or a G4 Mac mini with a modified Mac OS 9 install are still cheaper, which will always be faster, at the expense of compatibility with earlier apps that may only work on Mac OS 7 and not 8.5/8.6 or 9.