My VW T4 was a ex mobility Transporter and had windows all around and all fixed - ventilation was achieved by cracking open the cab windows
My VW T5 was the Shuttle model - so had windows all around but the two in the sliding doors were openers. But as they slid open, not useful in the rain so again it was the cab windows getting cracked open....
My VW LT was a ex-minbus - so yet again windows all around and this time just one window had a small top section that slides open.
But for THIS build, and from what I learned from living with the previous VW builds, one of the very first things I bought before I even started the conversion was a Maxxair Deluxe Fan and I can't recommend it enough.
With the cover down it has a small amount of ventilation possible; The cover will raise and the fan can be used in the rain and when driving; The fan will adjust its speed automatically depending on difference between actual and target temperature you set and will even turn itself on and off as required (this time of year I see it shut down and lid drops in the evening and then start itself up come morning time).
An expensive bit of kit but worth the outlay and would be an automatic buy for any future build almost certainly.