Definitely plan on replacing anything porous like carpet and sealing/painting everything else.  

We live in a dusty area, so maybe checking door and window seals could help resuce the duct and, thus, the cleaning.

Our property needs a small amount of work (which we're currently doing - normal unit turnover work and a driveway sealing). 

My second choice would be sealed-combustion natural gas furnaces sized appropriately. 

Who repairs a leaking toilet via copious amounts of fleck seal.

Make sure your insulation, weatherproofing, and framing are up to snuff - a lot of garages are built cheap and drafty, and you'll definitely want to address that before investing in finished space.Drylock/seal the concrete.

@Benjamin Seale Thanks!

On one multi-family I purchased the seller was supposed to have had the windows puttied/sealed and sash cords repaired (amongst other things).After the closing I discover most work had been done but the window work hadn't. 

Once approved by the court the lender will approve, and I believe it goes back to court one more time to give the final seal of approval. 

Luckily the seal-directed retirement industry is quite large so you have plenty of companies to choose form.