I had one. The most annoying thing with the CB350 twins is the air filters. They're not made anymore, so the option is either a K&N Sock and rejetting, or taking the old filter, burning out the filament then taking a modern car filter, cutting it in half and gluing it into the plates.
Definitely take a compression gauge with you. I believed the guy I bought mine off of (bad idea) since he was a friend of my Mom's. Pretty much if it's below 140PSI, IIRC, you need rigs. Ideally in the 160-180PSI range. Mine was 110 and 120PSI respectively and was a PITA to start.
Find out when/if the oil change was last done and check the drain plug. It's common for them to be overtightened then rounded off.
Other than that, the 350 twin was definitely a fun and fairly light bike. Nice and torque and the typical twin vibration. Very different from the fours.
For now I'd be inclined to skip that one and look for another. Just looks too rough.