Definitely get a 3.5T van, you'll probably still end up overweight. I've got a 4m wheelbase ( extra long), and use all the space. My son has a med. wb and likes getting into smaller spaces, but we don't carry portys ( well we did for the first6 months or so). I'd always get a merc or VW, personally. Son's is a yucky Citroen.
Costs? Typically £25 / day on LPG, and a service costing £100-£150 every 100 hours, which is about 6 to 8 weeks. Lower chem costs than porty.
The only TM's I've got experience of are liquid cooled blue lines, but I'd go for the biggest engine I could get. They're no more to run, quieter, and will twin tool when you want it.