If you can get a used poptop easily for that price then buy it anyway. You can resell it in a day and make a buck. Used SMB poptops are
HARD to find and people will pay a premium, especially since the price of a new one from SMB and CCV has doubled in the last 4 years.
I have a 2015 Transit and absolutely love it. I'm hesitant to sing it's praises too much here since this forum is 80% diehard E-series fans, which I am too. I've had 4.
First, if you want a hardcore rock crawler at a bargain price stop dreaming of the 7.3 and buy a V10. Amazing power, MUCH quieter than a diesel especially considering your family road trips, virtually no maintenance. Read that again and think realistically about diesels. Yes, I love diesels. We all do. They sound amazing, give you dreams of conquering Everest in your camper, have seemingly endless torque.....all that. But they are not maintenance free. How much time to do you want to spend rehabbing the mechanical side of the van AND building it out before you can get in an enjoy it? The 7.3 hasn't been produced in 15 years! The V10 largely skips that first part and you don't really pay any more for fuel than a diesel, you pay it in oil changes and additives instead with the diesel.
OK, enough of that. Transit: It's not an old body on frame van. It's very Euro. It's Ford's answer to the Transit and they are beating them at their own game. Firm ride, firm seats, amazing handling, everything from bumper to bumper 25 years newer designwise, much safer, goes and stops and turns like a sports car, is SILENT in every way, has 6 cup holders for the front seats alone and better vision than a greenhouse. All that and it still manages to be truck enough to keep your manly nature happy and still get 16-17mpg. The Ecoboost is no joke. Go drive one. You can get a decent used one for 25k and then you have square walls to build your interior on. It's like a gift from the van gods.
I'll be curious to see what you decide on. The good thing here is that there are really no bad options. Oh, a low roof Transit, that would be a bad option. Why? Why do they even make them?