Then I saw an add at Menards for shingles on sale that were being discontinued. 

Seems like they are raising the price with a computerized counter bidder.

(They hope you will hang your shingle with them once you pass the exam.)  

The existing asphault shingles are weathered and worn, some shingles are missing entirely, what is the estmated range of cost to reshingle the entire roof?

Total costs:Roofing materials and shingles:  $1,344.56Windows (13) and materials:  $1,684.90For the roof, I went with Atlas Castlebrook Laminated Architectural Shingles in the color "Hearthstone Gray" which is a darker gray color. 

Now the only downside I see is it does have the asbestos concrete shingle siding; which I know some won't like, but even it's in really good shape.

Both buildings have flat membranes and then the old school shingle look on the outer and upper portion of the exterior  as can be seen in the photo.  

Flat roofs can be much more expensive to replace than pitched shingle roofs. 

I guess I have some research to start doing, for a shop to hang my shingle with.

I personally love robin egg blue, but unless you're completely replacing the roof, I'd pick something in the maroon/red family to match the shingles.