@livram
You would have to remove the old tile.
For the roof material go with the Bluescope brand of the color coated rolled metal.
You can opt for the either grade:
Colorbond - more expensive, thicker coating, I think 30 year warranty
Zachs - less expensive, thinner coating, 10 year warranty
Don't settle with the cheaper Chinese or Korean brands of metal, go for the best stuff - the difference in price is not all that great.
But if you're on a real tight budget then those should be OK also.
Make sure to get PU insulation.
Most of the the roller-forming shops that sell those roofs have machines that add the insulation layer during the production process.
25mm of PU insulation is sufficient - it's really great, but the thicker layer would be even better.
Bluescope shops are franchises.
Just about every sizeable city has one.
I bet you've seen this logo before.