What slope? What roof features? Strictly gable to gable, or are there hips and valleys? Not only do the complexities add to the labor cost, but they factor into which type of standing seam roof you choose.
McElroy Metal has a roof system that's designed to be installed directly over the existing shingles, with no tearoff or new underlayment needed.
Good points: Cheaper because there's no tearoff labor or trash disposal. Concealed fasteners. 1 3/8" seam height MIGHT be more aesthetically pleasing than higher or lower seam heights. Having the roof panels raised up off the shingled surface leaves an air gap from the eave to the ridge. In the daytime, the metal panels heat up, but the hot air vents out the ridge. This keeps the underlying shingles and decking cooler, which may reduce your summertime cooling costs.
Bad points: Not tearing the shingles off means that you haven't inspected the existing roof sheathing for deficiencies. You also haven't re-nailed the existing decking to bring it up to more stringent code requirements (if needed).
Depending on circumstances, it's USUALLY better to tear off the old roof before you put the new roof on. The main reason is to check the roof sheathing as I've noted above. If you do choose a full tearoff, I prefer drying the roof in with a 15# felt before installing a quality self-adhesive, high-temp underlayment like Carlisle WIP 300 HT (my personal choice over any of the Grace lineup of products). Whether you choose a full tearoff or a roof recover, this is not something a layman should consider DIY-ing (unless they have a lot of experience, or have friends with a lot of experience that will donate their time and expertise).
Something I recommend to people is to go to your local metal roofing supply stores and ask them what standing seam roofs they sell the most of. You can also get recommendations on local roofers that have a good reputation. The worst thing in the world is to just call a dozen roofers and say, "What do you recommend?" You'll get a dozen different answers, and then it's up to YOU to try and decide which one of these roofers will not only give you the best product for you needs, but also give you the best installation. If you go into it with a good idea of what you want, then you're more likely to get more apples-to-apples quotes.