Valley,
Roof warranties are not what you think they are. First off, like flooring warranties, they are impossible to get honored. Secondly, even if honored they are prorated. Thirdly, is the warranty the metal roof failure not installation. Meaning that if the metal roof fails at year 10, the Amish expect you to contact the manufacturer of the metal roof and request new metal roofs at the cost of 75% discount (10/40), the Amish will recharge you for labor. More importantly, the metal roof will not fail, the underlayment is the weak spot. The Amish will tell you that your house needs to be fixed, not the roof and you will be back to square one. A roof should be one and done forever problem.
With all that said, just slow down, put the tarp, and get estimates with and without shingle removal. It's a fact that shingle removal is the correct way to roof.
However, I agree that the cost may not be worth it for a 45k house and you may just have to roll the dice and hope for the best.
Best of luck.