You're right, if the mount does fix the "misfire" and the thermostat is the heat problem, you'll be in pretty good shape for a reasonable cost. In addition, if the filter was installed without removing the gasket from the prior filter, that would likely cause a leak and a careful oil and filter change could fix that leak easily. I hope all goes your way!
If all that pans out, you may want to consider buying a lottery ticket.
Good luck!
-Rod