My van was a converted panel van and my experience was constant leaking windows and a lot of hassle. Any water getting in rots the metal surround. Decent window seals are hard to find and they only fit if the metal window surround is perfect with no blemishes and no rust. Being a panel van you can't stand up in it and it can do the back in. I would either buy the panel van and enjoy it as it is or buy a pop top with factory windows. You asked for lots of opinions and this is mine.
If it's any help in making your decision ..I've got a twin slider conversion ( done yonks ago) If I was making the choice now I wouldn't do the rear panels at all , they leak and scab up and look pants . One day I will weld up the rear panels but leave the windows in the sliders
Mine is a van with original curved triplex side windows (pic under introduce yourself). Agree that the the interior is a bit more challenging to get looking good (especially the slider). It's personal choice at the end of the day.