Would there be problems with moisture,or extreme heat issues if installing a metal roof over existing shingles.

No,You would install 2X4's spaced out for support of the metal roof,perhaps a 2X6 treated at the base where the most of the water would be(naturally), the peak would be vented,typical for metal roofs.

The ridges in the metal roofing allows for venting,moisture is the same if you install over sheeting anyway,there is just shingle below it.

I have BID jobs this way,and lost a few,got a few.. I see no big deal to leaving the shingles on.

Just my opinion.

The shingles just give your roof a little bit of extra insulation. You might want to add some insulation immediately under the metal roof otherwise you'll be awake at night when it rains hard due to the noise on the metal roof!