Also bear in mind if you are going to sell it then you won't get back the money you spend getting it in to tip-top shape. If you are doing it for the love or fun of it then that's great, but don't be surprised if you end up not getting a price that you think reflects the cost and effort of repairs you'll put in to it.
There are plenty of buyers that can get the rework done themselves if you are upfront about what work you know needs doing.
As for the airflow, the fan is a good idea for troubleshooting.
The mask of the CPU (version number) will be an important factor - what are the numbers printed on your big 040 CPU? See here for more detail
Simply swapping the 040 for a newer mask if you have an old version will reduce a lot of heat. Since you are in the US they might be easy to source. An 060 CPU in an adaptor will run even cooler (as it will run at 3.3V rather than 5V) but you will need to burn and install a modified kickstart.
But the better option is to do all the work you want to do and keep it for fun