Is $300 fair for a "not new" but un-used SMC kit?

Depends on how many upgrades - how current. I paid $180 & need 5 upgrades - it's been laying around foe almost a year. I ever get the protection I need in case it fails finished, I'll send it in for the upgrades.
There have been several updates to the system in the last year. You can get the system for around $300 on a group purchase. If the setup it current, then $300 is about right. Its usually around $350 normal price.