Best way to buy a van, (unusually) I completely disagree with Spruce on many points.
Leasing/contract hire is an absolute No NO, from paying Vat, over priced, ridiculous mileage restrictions, to the final bombshell, them charging you for a complete respray on return, for a few small scratches (and it will get these),
Leasing is financial suicide,
Buying a better van that what you need is also a money trap. as is buying a van on finance. You lose both the depreciation and the interest on the loan.
You need to pay CASH (what you can afford), get the best bodywork but don't worry too much about higher mileage as for our work, high mileage is not an issue, having six working doors and a bit of size to carry and access equipment if more beneficial, a slightly bigger van than you think you need rarely proves a negative.
If you dont know a lot about vans take someone with you to look or have it checked by an independent mechanic,
For the fittings you will do a cheaper job by doing it yourself but finding someone to fit the tank/electrics/poleholders.etc and paying them, will get a better job done.
Whatever type you chose value for money will be the overriding objective, dont consider resale value as if its Ok you will probably keep it till it is no longer viable .
Signs to look for- scruffy interior and torn/damaged seats =Big Nono
Service history =good
Clutch, all our vans have needed a new clutch at some point, if the prospective purchase has had a replacement fitted recently that's a benefit, as the down time and inconvenience is annoying,
Tyres, 4 good tyres, preferably of the same make good sign the vans been maintained professionally.
The van market is competitive so Autotrader is a good start, just don't tell them you want to pay cash as retailers make as much money from the finance and part X as the sale, (I have been known to string em along by asking about finance to get a discount) you been Scottish probably will know about this

Happy huntin.