@Dori Rutledge
If the decision was mine to make, i would have the owner replace the furnace(s) and the window panes. However, I would want to know is it one furnace for all three units or does each unit have its own heating system. It sounds like it is just one furnace.
I would want to know what type of furnace is he replacing the old one with. Would I be able to choose it. With such a wide range of furnaces on the market today, you want to make sure that you are getting one that is reliable, energy efficient and has solid reviews. Who is paying the heating cost? You or the tenants?
I would also want to know how many times the roof has been re-covered in the past. If it is shingle, how many layers are on it? If there are too many layers, the shingles may have to be removed and replaced altogether and not just re-covered.
The reason that I would go for the first option is:
1. A new roof is definitely low maintenance. You could make that replacement purchase yourself and be nearly worry free for at least another 20+ years....and it comes with a longer warranty than a furnace. People usually don't call up roofing companies for yearly service maintenance.
2. A repaired furnace would most likely need another repair by the next winter.
If it were replaced with a brand new furnace, the only maintenance that you would have to contend with are the yearly service calls at the beginning of each winter. If it is cared for properly, you should be worry free.
I installed a brand new roof and two new furnaces for my duplex 10 years ago and have been worry free on both of them. The heating company only comes out once a year for general maintenance.
Hope this helps but there is more information that I would want to know.
Missy