As they were moving in someone did something to the electric meter killing the power to the house and then they stole the AC condenser with the tenants in the property.  

My realtor had an AC condenser stolen in the neighborhood. 

Some of the windows are rotten, and there are sliders where condensation is on the inside so those would need to be replaced.

Understand that when buying cash your only advantages are speed and simplicity/lack of appraisal -- therefore try to keep your timeframes in the contract as condensed as possible and make your contingencies few and far between.

I'm just not techie enough to call up past posts that address this area of RE, sorry, I guess it will need to be condensed into a blog so that every time the same types of questions are asked (which seems to be at least every week) they can simply be addressed to some blog.

The "leak" is the normal condensate that is removed from the air during the cooling process.

Note; paper underlayment will be cheapest, but it's cheap for a good reason, and it's not its robust life expectancy...The strapping you mentioned is a slight added cost, but if done properly (vents at the eve/drip edge and ridge), can actually help lower inside attic or top-floor temperatures, as you now have an "air gap" to help alleviate some of the penetrating UV heat, as well as helping to drain and dry condensation that WILL form on the backside of a metal sheathing.

They both will no longer do work at the property as the tenant is negative, condensing and rude/ nonresponsive.

However, with it being 23 years old I would go ahead and replace furnace and condenser.

A NOTE ABOUT 90% FURNACES IN UNCONDITIONED ATTICSThe cost savings on the standard 80%s were NOT the only reason we installed a 80% in the attic.   90% Furnaces produce WATER during combustion which must be dealt with, i.e. pumped out on condensate lines.