In reply to coinneach:
All of what ExiledScot said.
Check for rust. T4s are all old now and they do rust. It very easy to end up spaffing a lot of money keeping on top of rust. Budget for major repairs, every component on any vehicle that age and mileage can potentially fail. Especially think about the gearbox and check when the clutch and DMF (if it has one) were last replaced. Same for cam belt and water pump. Also check servicing. I assume you have looked at many others for sale online and have an idea of what a reasonable price is. These VW vans are solid and parts are relatively inexpensive. There's a reason why there are still so many old ones trundling around.
Check out the heater if it has one. Make sure the seller can fluently answer any questions you have and if not be suspicious. A badly fitted or faulty heater can be a death trap, or you could end up spending a lot of time/money refitting it properly. Assuming it's been self converted, you can get a good idea how good it is by the general tidiness of visible wiring, plumbing etc.
With regard to layout, don't get too hung about it as you won't really know what works for you until you have used it for a while. I had a T5 hightop for a few years and loved it, but all told if I have another it will be a pop top. That's just me though.
Post edited at 21:27