First thing to consider is price. Is the price of the 'nearly new' van sufficiently far away enough from that of a brand new model? Because for the sake of a couple or three grand you may be able top get into a brand new one.
The main problem you'll face is the fact the the next owner would be the 3rd in the log book. That's fine if the vehicle is around 10 years of age but, if you sell it after a couple of years it isn't going to be as desirable on the used market as a one owner vehicle and the used price would reflect that. It wouldn't be a massive amount, up to a grand maybe, but it would take longer to sell as a result. Also consider the price differential of a T6 and T6.1 registered at the same time. It's likely that you've been quoted for the T6 (I'm assuming so) and this means that used values will take a hit when the two models appear on the used market for the first time. Buyers will always gravitate towards the newer model.
Like I said, make sure the price is far away enough from the cost of a new model. If there isn't a big enough value-for-money gap, hang out for a new one. Also, go and get a quote for a new one, rather than just looking at the list price (there's always a discount available). That way you'll be able to get a better sense of the value-for-money this current dealer is offering you.