Having bought several vans for different people, the size is down to your intended use for it. LWB for the extra length, high roof if you want to stand up straight in it, or carry tall furniture etc.
My son has a LWB Renault Trafic (same as the Vivaro, or Primastar just different badging) which he uses for his carpet fitting work - this was the only van in that size range that could accommodate a 4m roll of carpet, without going to the LWB Crafter / Sprinter size, albeit the 'bolt in' bulkhead panel and front passenger seat had to be removed with a gizmo to make the van think the seatbelt warning was still wired in! Nothing else had the length capacity.
My daughter has a LWB high roof VW Crafter, which is a great van; exceptional length and height - I am always borrowing it! Nice to drive, although can be a little gutless with a full load - the identically bodied Mercedes Sprinter has a more powerful engine, but tends to command more money.
Another friend has another VW LWB Crafter high roof for his work, and likes it.
There are some good purchases to had at auction in van sales, if you are prepared to hunt out the good from the bad. Don't forget the majority of sales at both auctions and dealers have VAT added on to the sale / displayed sale price, which on a seemingly good deal, results in it looking costly.
I have found ex Enterprise Rental vans very well maintained, and generally in good order overall, unlike builders vans, or maintenance vans which seem to be worked hard for their money.
Most vans tend to suffer on the drivers seat if the van has been used for deliveries - the continuous in and out of the driver knackers the sides of the seat base and upright. If the drivers seat is ok in any van you look at, then it's a reasonable indication that it has been used on longer runs rather than stop start deliveries etc.
Usual pointers apply as with any vehicle you buy as to its bodily condition / mechanical condition etc - and if in doubt take someone who wouldn't look at it through 'rose tinted' glasses and would give a honest opinion.