In-ground oil tanks inevitably become issues when you go to sell your house. Worse, it often comes up post-inspection, i.e. after you've agreed on a sales price. Most attorneys recommend that buyers do NOT buy the house with the in-ground oil tank and recommend that the seller either removes it or decommissions it.
The reason for this is that liability for any leaks, etc transfer with the property and, if something goes wrong after closing, it can cost the buyer a lot of money and aggravation.
My advice to you is to first deal with the oil tank and then look into upgrading the furnace and/or converting to gas. This also depends on how old the furnace is and how much life it has left. The age of the furnace is more relevant than the type of heating fuel.
In this area, central air is a plus, but not a deal breaker. However, if the house is hot throughout the summer, you might want to consider retrofitting an air conditioning system with an outside compressor and
a mini-split system to get cooled air into different rooms.
If you want a recommendation of a company to test your in-ground tank, please contact me.
Coldwell Banker Village Green