My name is Tyler Canning. I have been roofing for over 10 years and my advise is reputable first of all when redoing your roof it is not a good idea to put more than one roof on over top of each other for a couple of reasons.
1.over weight on truses ect. 2.dual heating action. 3.life expectency decressed. Now reasons being for this is simple because you are re roofing your home when straping your roof for the tin or if it directly applied to the old shingles becuse of dual heating in the summer or what ever the case may be, the strapes will eventually twist or buckle because they are under load, thus costing a great deal to fix, even from a reputable company with there warranty,alot of tin roofers will disagree with what im talking about, but of seen this happen time and time again it is in your best interest to remove the old shingles it might cost 1000 dollars more but like you said you want a roof that will be maintenance free do it right the first time Good luck. Tyler Canning