Metal roof installed over shingles

[April Birdsong]

Rehabbing my 1st rental. Is it ok to place metal roof over shingles?  I have done some Google research and articles point out concern for not being able to see the decking and determine if there are issues and fix them. 
On inspection, he said roof likely had 5 years left. Contractors said shingles are pretty crispy etc. 

They have quoted me $7000 for metal roof. House is $1380 sq ft. 

Have any of you done this?  What to look for and be concerned with. I know I can wait  a little longer. But seemed like a decent deal and once done I would have a replaced roof. 

Thanks for your thoughts. 
April Birdsong 

[Henry T.]

I remember reading the metal roof literature saying that you can. I however would not. I want an all flat surface. No possiblilty of dents if you walk on it.