It's a great find, and modifying it will be good.
Hardware cloth flooring won't have chicken poo falling through it, and will have raccoons pulling chicken toes off . Not good, either way.
Are you going to have an 'all-in, all-out' operation, or add and subtract some every year? I think that getting no more than fifty (easily raised!) this year will give you time, experience, and room for more next spring, which will help keeping egg production more even, especially in winter. Commercial operations have replacement pullets raised in a separate building, so when the 'older' one or two year old birds are all eliminated, the new ones are already producing. I think planning on a rotation of about 1/3 each year might work out well, if you really want constant egg numbers.
How long ago were chickens in this building? Marek's disease lasts at least one year! Mycoplasma is gone in three weeks, but I'd be very concerned if birds were there within the last year. Then, very serious cleaning and disinfecting are necessary!
Mary