How much would you spend on it?

It's a money pit if you need a lot of work, like @Princess Valiant said. Take out the window and regulator, and pound out that dent with a block of wood.

Not running, I've paid $500 for a car in similar condition, but engine was seized. I made almost $2k from the parts I sold, and I have a solid floor pan and trunk for my other cars.

I'd go $1500 considering it a parts car, then see if it ran and go from there.