Hi Tony,
You won't have any issues with installing OEM sliding windows into panel van and you will not change the integrity of the structure in any way at all, I believe this excuse is given by companies that don't want the additional effort to fitting OEM windows it only takes a little bit extra care but very achieveable especially when you have engineering background, I have never had any leak issues either. I will send on some pics to you with measurements no problem later this evening
Regarding the cost, when I prices it was much cheaper to buy the sliders here in Ireland off main dealer than they were in U.K. For some reason. There was no cost difference between the green and tinted
I will send cost details later also, if you need anything else just let me know