Yep that's a good frame made of 531 DB, probably rides real nice. BTW if you're interested in fat tires, those usually fit 40+ mm, like Hetre or Baby Shoes on 650b wheels, and RAID brakes will reach. Makes a comfy all-road bike.

for rust removal, I'd probably use Evapo-Rust or one of the DIY equivalents — example:
though oxalic acid may be less expensive. Electrolysis can be great too if you have a DC power source like a car battery charger.

Evaporust is expensive to buy enough to cover a frame but you can use a lot less if you build a bike frame shaped "tub" with a little lumber and some plastic sheeting. I think there are threads here about that. Search on oxalic bath I think.

If you're too lazy to build the tub (as I am), a plastic tray for hanging wallpaper costs like $2 and works for all the tubes except the seat tube, which can be done with Evaporust gel that sticks where you put it.