I think the price depends on whether the Perkins is any good and therefore it is dependent on whether it runs. The Perkins is definitely more durable than the F head but is also way more expensive to rebuild. Power wise, not much of a difference.
The tub appears to be cobbled up rather well with the hard top welded to the windshield frame and perhaps the tub also. Homebrew? If you just want to run it as is, that may not be a problem but if someplace down the road the plans include a restoration, it is going to be a lot of extra work and probably a new windshield frame. Rust? From the photos, it doesn't look too bad but you should check it closely. If the floors and hat channels are still good, the rest is likely ok. They typically rust in all the same places as the flatties: sills, floors, rear body mounts, and the sandwiched rear cross member.
Price? Without actually seeing it run, I definitely wouldn't go over $1000. If it is rusted out and doesn't run, your $500 may be on the mark. If it does run and is largely rust free, maybe $2500. As noted, it would still require a lot of work for a restoration.
