I've got a Westy T4 California 1.9 TD and it's fab. New ones, as you say look great but you pay a massive premium for them. Westy T4 layout is almost identical to the VW T5 Cali, but much better finished IMHO. For that reason I'd personally be looking at a late model 2.5TDI T4 as an upgrade (there is a synchro version too, but pretty rare).
I wouldn't overestimate the amount of size inside. With the rear seats / bed folded flat you'd get a couple of bikes in on their side but you're going to need to be really careful to avoid damaging the interior and upholstery (put tarps down etc).
You've got space in front of the rear seat but in my experience I can only fit one fully assembled bike diagonally between the rear and front seats (the fitted units take up about a third of the width). For that reason I tend to have the bikes on the rack all the time
I also reckon it would be a huge hassle to keep the bikes inside overnight and sleep in the pop top. Apart from anything else you're probably going to need to take them out before making a brew in the morning! At the very least they're going to block access to something vital in one of the cupboards.
Pop top doesn't have a huge amount of room and getting in and out can be a bit of a mission if you're both on the <ahem> larger size, unless you're happy with your head at the thin end of the wedge, so to speak.
For that reason we always have the bikes on the rack. Locked through the tow loop. Occasionally will pop the bikes inside if we go out for the evening but we always sleep downstairs (our little boy goes in the pop top!). You're head is only 6 inches away from the rack so you'll hear if anyone tries to nick your bike.
For what you want, you may be better with something like a multi-van or Combi (perhaps LWB?) with a pop roof conversion rather than a full camper. That would give you the space and flexibility you want plus the ability to sleep in it.