You may want to have your AC/Heat system checked to make sure you don't have condensation forming on the inside of the windows.   

Here are my condensed thoughts on getting into this space: 1.

@Mitchell Litam if it's an exterior wall like I gather it is, it looks to me like condensation?

When a property built in 1988 has the original air condition condenser, it is a safe assumption that the cap ex will be cashed in sooner rather than later.  

The other downside is since it is a dedicated FA AC system, the vents need to be covered with magnet covers in the winter so ducts don't condensate in the months it's not in use.The old steam radiator heat has been wonderful!!! 

I have done the same thing on my rentals in getting the big $ items done up front. my second rental I am in the process of purchasing has a really old condenser (coil is 2 years old) so I will find a good deal and do it now, and not sit and get a call on Christmas morning that it broke.

Usually, only heating oil USTs associated with commercial properties are regulated. thus, most home heating oil tanks may and often are taken out of service without any oversight from state regulators.Second- the rules that are in effect for heating oil tanks are usually less stringent than for diesel or gasoline tanks since heating oil tends to thicker and may not migrate as far. as a result, many states just require tanks to be cleaned out. they may also require the tanks to be filled with sand or concrete to prevent collapse. but sampling may not be required.Third- the key to determining if a tank has leaked is to collect soil samples from around and beneath the tank (groundwater also if groundwater is shallow). another quick way to see if a tank is leaking is to "dip" it with a stick that has a paste that turns color in the presence of water. generally, if water has gotten into a tank, it can mean oil has leaked out of the tank (although a certain amount of condensation may be present even where there is no leak so this is not a infallible test).Fourth- if there is currently a tank in the basement, ask the owner if there used to be a buried tank (a/k/a underground storage tank).Fifth, If the tank has impacted the soil, the cleanup generally varies from $25K to $50K though the costs will depend ont he depth of the contamination. i once had a home with a heated pool that had pressurized piping and used diesel. the contamination went down 40 feet and the consultant went crazy excavating the soil to the tune of $400K!!!

Double pane are way more energy efficient, but they have an expiration: is there a fog or condensation between the glass? 

It will probably have prexisting hvac so if you did replace it would not need to be ducted which doesnt include the needed electrical which could often include upgrading a main disconnect or running 220s to a disconnect at the condenser or flex line for a furnace in the case of an old multi in Baltimore on radiator. 

This is a condensed and over simplified example, of course, but hopefully it answers some questions.