the roof needs a complete overhaul due to sagging, sinking, and the shingles have no more asphalt on them to protect against the weather. 

In the interest of future ownership and financing, new asphalt will be placed this spring.

We’re also capable of performing concrete and asphalt work in-house.

Are you looking at asphalt shingles? 

Definitely a fixer upper, like $25/foot for interior, exterior, landscaping, asphalt improvement.

When you go to sell it the potential buyer(s) will almost for sure have a home inspection done and then they will probably want you to replace the roof so you might as well do it now. 15 years is along time for a roof (assuming that it is asphalt composition). 

Leaking toilet with ceiling repair needed, another leaking toilet with sub floor repair needed, some new windows needed, new AC needed, a bunch of small electrical work, some roof issues due to ice dams that needs to be fixed, asphalt and garage foundation repair, gutters needed (none).

I know that copper lasts longer than asphalt shingle and is more complicated to install.

I'm not sure what type of roofing it is but in my area (northeast) the most common is asphalt shingle.

Do not put a tile roof on  a house where all the houses have asphalt roofs.