Converting corn crib for alpaca housing?

Hi,

I'm hoping to adopt some alpacas later this summer, and I'd love to be able to transform this old corn crib into housing for them.

Obviously I intend to sand and paint to get rid of the rust, and replace the missing roof panels...but what about the floor? I'll reinforce it from the bottom, but what would make good flooring in this situation? I was thinking rubber stall mats. Could there be any benefit to leaving the grated floor, and just putting straw over it?

Also as far as the grating, I'm not to worried about the walls right now in summer, but I think I should probably do something to weatherproof it before winter.

I'm open to any suggestions, or pitfalls you'd like to point out.

Thanks!