If you're thinking of buying a base van to convert to your requirements, choosing the right one is both critical and confusing - I'll try to demistify this for you.
Short Wheel Base - the most common platform. Not much bigger than a family car and easy to drive/park Long Wheel Base - 40cm longer which doesn't sound much but does feel significant in terms of interior volume. Less common and can be tricky to park (but you get used to it)
Panel Van - your typical builder's van with a bulkhead and no windows at the rear. Single side door usually. Good for conversion but most work and highest material costs. Kombi - with seats in the rear and load space behind. Factory side windows fitted. Good for conversion with less work involved once the rear seats are out. Shuttle - have multiple rear seat configurations and factory trim throughout. Tend to have been used as taxis, so beware! Not good to convert due to having to remove so much trim, ventilation, lights etc. Often have twin sliding doors which don't really lend themselves to camper conversions, although plenty are. Caravelle - the luxury people carrier. I would leave these as they are, they're lovely!
T5 - 1.9 and 2.5 diesel engines. Expect high mileage and ongoing running costs which can be significant. Pay up to £10k for very good examples T5.1 - from 2010. 2.0 engines in various power ratings. Avoid anything lower than 114bhp, ideally 140bhp. There are lots of basic 102bhp vans out there which really don't have the power for comfortable touring and only 5 gears - avoid. Definitely avoid the 180bhp BiTurbo models which can suffer from very expensive oil consumption issues. Later T5.1 models may have Euro6 bluemotion technolgy if you want to be green(ish) Pay up to £19k for the best examples T6 - 2015/16 on. Similar to T5.1 but all euro 6. The 150bhp model is ideal. Pay up to £28k for low mileage, good spec van
T6.1 - 2019 0n. These are very similar to T6 but have external and internal enhacements. Expect to pay at least £20k for basic models - again, best to avoid the lower powered models.
The 6 speed manual gearboxes are all good and strong. 5 Speeds (on low power models, so avoid anyway) are ok but less reliable. Early Autos are ok if well serviced. Later DSG autos (6 or 7 speed) are lovely but expensive if they go wrong so servicing is critical. DSG models will come at a premium.
Model load Ratings:
T26, T28, T30, T30, T32. This relates to payload. Look for T30 or T32 which will happily carry the weight of the conversion parts and leave a safe margin for extras. Specification Levels (generally): Startline: basic, no air con, black bumpers Trendline: Better instruments, air con, body coloured bumpers Highline: Nice, car like accessories: multi function steering wheel, cruise control, trip computer etc Sportline: T5 - very nice. 174bhp 5 cyclinder engine is a beast, lovely styling. T5.1 Avoid - these have the 180 BiTurbo engine which isn't worth the risk. T6 - lovely if you can afford them!
Dual Mass Flywheel (DMF) - all have these, even autos. They tend to start to fall apart at high mileages and where the van has seen lots of stop-start use. Listen for obvious rattling on idle or vibration through the clutch pedal. Higher mileage vans will probably have had these replaced. Be careful if not. Driveshafts - these wear fairly quickly. Any clunking on pick up or slow down would suggest they are nearly shot. EGR Valves - as with most vehicles, these can fail and cause issues, lack of power, smoke etc. Check for warning lights on the dashboard. Diesel Particulate Filters (DPF) similar to EGR valves - any lights on the dashboard, walk away.
In brief, my recommendation is:
140 or 150bhp
Ideally DSG if it has full service history and sensible mileage
Service history - more important than most vehicles, look for comprehensive, documented records
For ULEZ exempt vehicles, you need T6 (2016) onwards
Any colour but white!
There are of course many other detail variations but this is a good start. Happy hunting!