IMO, the seller's asking price is too high for a trailer that has been modified to that extent. I would keep looking. The thing is, w/o a black tank, you are pretty limited to full hook-up RV parks. Messing with a blue tote for black water is not something I want to do on a regular basis. We have done it in a pinch - but if that was the "norm", I'd find myself not wanting to camp much.
But if you can work on the price and decide it's "the one" for you, you can add a black tank back into it. You will need to drop the belly pan and remove the toilet. If it was replaced with a residential toilet you will need to get an RV one. The tank itself can be purchased new or custom made. Then it's just a matter of putting it back together. We added both black and gray tanks to our '56 which didn't have any. We did a full restoration including taking the floor out - but I don't think you'd have to go to that extent unless the floor is rotten in the bathroom around the toilet.
Overall, it would be a pretty big "first" project for a new trailer owner. If you are a hands on project kinda person, it wouldn't be too bad. But again, I feel the asking price is too high for a trailer needing this sort of project unless EVERYTHING
else is 110% perfect. Good luck in whatever you decide to do ~