Hello Jason
I think Dibs is right.
I'm retired so I'm time rich and cash poor.
If it was mine, I'd recover the slate and see how much was reuseable. Take one off and make sure it's a standard size that you can still get locally.
Then felt it and replace the laths. As long as you get the lath spacing right, fitting the slate isn't that difficult and it's only a single storey so scaffolding it shouldn't be too expensive, if you don't fancy climbing off ladders.
One thing to think about is how flat the plane of the roof is. From the pic's it looks a bit uneven, which is OK with slates, but a big flat sheet of metal might be a bit awkward.
Good luck with it, whatever you decide.
Cheers
Dave