Love efficiency and MX-5s, but have never combined the two - fascinating project
The number one rust spot for mk1 MX-5s is the back of the sills and the rear arch. Have a very thorough poke around that area. They rust there from the inside out (usually due to blocked hood drain tubes causing water to collect), so rust that becomes evident is already structural.
On the plus side, the engines are very understressed (fairly low compression as it was designed to be turbocharged, apparently) and smallnose crank wear notwithstanding, are extremely tough.