Look out for a Saturn that has round buttons. They have 21 pin ribbon cables from the CD board to the main board, and that's the only type you can still buy mods for.
I'd buy a mod from GameGizmo
if I were you as the mods from Lik-Sang
may have some compatability problems.
Saturns with a 32 pin IC on the CD board are better to mod as all mods seem to work fine with them.
Saturns with a 64 pin IC on the CD board seem to have compatability issues with Lik-Sang and Lan-kwei mods, but a few have reported success. Gamegizmo mods seem to work fine with 64 pin IC Saturns.
There's a rarer type of Saturn with an extra PCB (called a PC trap) in-line with the ribbon cable. Only one person so far that I am aware of has successfully modded one of those.