Test drove a low mileage ‘99 boxster today. Noticed a slight but noticeable vibration through the seat at idle. Normal or a sign that engine mounts are needed?

Yep, if original, pretty much guarantee it’s shot. Not expensive, and relatively easy to do.

If you drive it again, see how it feels around 3k, also drop the clutch a couple of times and see if you hear/feel a good clunk, pretty good indicator of a dead MM.

Vibrations are tough though to figure out, there are other things that can cause vibrations.