preamp - price vs age

If it was working all these years and tubes replaced promptly, it shouldn't be an issue. Otherwise DC electrolytic caps might need to be checked/replaced for circuit safety reasons. Make sure to mark + - to avoid misconnecting.
For power supply and transformer I would recommend inspecting mounting tightness and replacing or adding soft washers or silicon between the chassis and mount.
Signal path caps less-likely need to be replaced.

Finalize your work with resoldering all joints with fresh solder if you're savyy.

For me it's worth. I usually tax the same amount if I pick up equipment to fix or to service (mainly studio gear and live sound)