I would stay away from a pop up roof" the beauty of the high Promaster is you don't need a pop up, it's not a Vanagon! Why burden yourself with something you do not need and really has nothing to offer you but potential problems and needless cost?
KOV is right. I had a Westy and loved it but it was really too small for extended travel and had too little storage. Then a pop up truck camper for 8 years. I bought a 136” High Top and WOW! this is the way to go! My diesel gets better MPG than the Westy did and has 23,000 carefree miles, a year of camping trips of 2 weeks to a month long and I am Happy, Happy, Happy! No more pop top for me.
But to come back on the other side... if you really do want a pop-top, that is an option. Can't remember who at the moment, but someone here has built out a few basic low-roof PM vans with pop-tops added.