As far as the bikes are concerned, no way are you getting two bikes in with the bed down without seriously messing with the upholstery, with the possible exception of an LWB conversion. You might just squeeze two in with one behind the front seats and one on top of them, but that leaves you exiting by the back door.
Not sure if it's that much of an issue though to be honest, why not just leave them inside when the van is unattended and lock them to a rear mounted rack when you're sleeping. If anyone tries to nick them, you're going to know about them and in a perfect position to give the thieving scrotes a thoroughly good thrashing with a rolled up copy of Camping & Caravanning monthly.
As far as engines are concerned, are we talking T4 or T5?
I've got a 95 T4 Westfalia 1.9 TD. Despite putting out only about 68bhp. It goes fine, and will cruise all day on the motorway at 75 but can struggle a bit on hills when it's fully loaded without a decent run up… you have to drive it in a bit of a singlespeed style if that makes sense. Personally I love it.
The 2.5TDI is loads more powerful, but most VW mechanics seem to really like the 1.9TD and it's far cheaper to fix if anything goes wrong. You'll certainly get more power with a 2.5 TDI, but will pay a premium. Horses for courses basically… both decent engines, but don't touch the non turbo 1.9.
T5s are a completely different story. The 1.9 TDI puts put about 102 PS I think, so I guess there is less of a performance gap.