Rust spots - worth a build?
 

I am a few weeks into my first Miata, found a 99 roller with large hole in oil pan for 400$ (no title) to start an endurance car build. Has ~290K miles on it, looks to be all stock. Based on my 2 days of research before buying it, and some sight unseen input from a few Miata guys, it seems to be in decent shape. Once I fogged it several times and got most of the interior out, I can't find any rust on the bottom of the car, the frame rails, or bottom of quarters/fenders/doors, but it has surface rust on both floorboards, the spare wheel tub, and some rust created daylight where the battery was. I think it sat outside with a torn top for a while.

I think it is decent enough shape to use for the base for endurance racing since the rust appears to only be in the interior, but would love to get input. Before I put $3-4K into the roll cage, I wasn't sure what the best approach to the surface rust is, while I am gutting it and prepping for the cage and painting the interior. I guess I could have the shop put new metal in on the passenger side and spare wheel hub? Or just por15 those areas? Wire wheel it first to see how bad it is? Sand blast it? I am not a body guy (yet).

Link to Google Photos album (hopefully this works, couldn't attach photos for some reason): https://photos.app.goo.gl/w2sY24B5DZU9i1Tb9

Liberal application of rust conversion paint and send it.

I couldn’t see from your pictures, but how does it look right where the front sway bars connect to the rails? NBs are notorious for rotting there — if it looks good there, por15 everything and send it!

Added shots of both sides, looks good except missing a few bolts there : )

That's about as rust free as you are going to find one that hasn't lived in either the desert or a garage it's whole life. Send it.

I would have new sheet metal welded in where the battery was, and near the dead pedal on the driver side, while you are getting the cage put in.
An endurance car will have many people in and out, and you want that area solid.
And it's always good to keep the battery in the car.
Otherwise it looks good, do it.