Interesting to hear all the negative comments. I never thought there was a serious issue installing a metal roof this way. You don't want to put the metal directly on the shingles, but with purlins/strapping, ok. Certainly its not 'the best', but for a diy job to save thousands of dollars, I would do it on my own house. If you're sloppy/drunk/don't care, certainly the whole roof can tear off, but the point is that this is your own house and want to do it well--- make sure the screws are screwing into something.
I put on my own metal roofs (screwed down ag panel type) on a new construction garage, as well as replacing a 65 year old 5V metal roof on my previous house. The old 5V panels were put on with lead capped nails, which about 1/4 were missing or uncapped by the time it was replaced, and the roof didn't leak by any amount that was noticeable. The nails went through the ridges.
Ag panels aren't standing seam, but the absolute best isn't always necessary. I've been told to ensure it stays leak free, screw through the ribs, rather than the flat section. Goes against manufacturer instructions, but you're not relying on the rubber washer to seal. Drawback is that it causes the screw to flex with expansion/contraction, so they'lll need to be replaced at some point before fatigue breaks them, perhaps 10 years is reasonable as preventative maintenance. My garage went 15 before I sold the house, and I only retightened the screws once in that period and none had snapped.
As for the purlins/strapping, make sure it goes into the rafters/trusses. Just have somebody stand in the attic while the top and near bottom rows are screwed in, make adjustments as needed, then mark for the remainder with a chalk line.
As to cost, 75% off for DIY sounds realistic to me. Hearing about quotes others received for their metal roofs shocked me. Material pricing (not including the wood) is probably about $2-2.50 per sq ft of building footprint, based on panel cost of $1.10 per sq ft right now. Panels were about half the total cost, and the remainder was screws/trim/etc. Current price is about 2x what I paid-- material costs are very high right now. The panel fabrication shop was just down the street from me, so I was able to buy from them directly.