I Did Not Budget For A Roof But May Need One - Do or Don't?

[David Healey]

I bought a flip property needing light cosmetic work (new flooring, cabinets, counters, appliances, minimal drywall and electric work, smoke smell removing and paint. I budgeted to do some of the work myself (swap out incandescent for updated lighting, all demo and haul away and install the cabinets and counters). I bid the rest out with a contractor which came out to about $13k.  After we got in and were able to see the roof, it looks like it's about 15 years old and has a couple layers of shingles installed. It doesn't look like it's leaking anywhere and the professional inspector showed minimal signs of past leaking in the attic but it was dry and didn't look recent. The lowest bid we've received is for $4,600 for tear off and replace with 3-tab asphalt shingles, $3,275 for another layer on the existing roof, or $2,975 if I strip and haul off the existing layers and replace any damaged sheeting. 

1. If it looks old but seems to be functioning fine would you replace it, or layer over it, or neither?

2. If you would replace it would you layer over it or strip it yourself and resheet where needed and have them replace. 

Thanks for your help!