Welcome.
I also went from a Caddy to a T6. Best thing I did - I loved the caddy but the transporter is on another level, so versatile and practical and it gets under your skin and becomes part of the family.
I’d try and get DSG and either the 140 (Euro 5) or the 150 (Euro 6) engine. If you live in a clean air zone or somewhere that will soon become one then avoid the euro 5.
The early 2015/2016 T6s were Euro 5 and Euro 6 started late 2016.
I would also suggest avoiding the 180 bi turbo.
If you want a Kombi then there are two options - factory Kombi or Kombi conversion from a panel van.
My first van I converted myself and my current van is a factory Kombi - the only difference is the side windows, and some say the aftermarket windows are better than the oem ones anyway as they don’t leak!
If it’s been converted you want to make sure it has been done properly, using genuine parts and has also had the logbook changed from 2/3 seater to 5/6 seater depending on whether you have single or double passenger seat.
You should approach it like any used vehicle, carefully checking the condition inside and out, looking for a detailed full main dealer service history. Timing belt and water pump should be changed at 80k/4 years regardless of mileage so make sure there is documented evidence it has been changed. Also get a history check like HPI.
Don‘t hand over any money without seeing and driving it first - this might seem outdated in the age of buying cars online but I still insist on taking the old skool approach.
With a van there are possible uses that might accelerate wear and tear like multi drop deliveries, builders or other tradesmen - try and establish who owned the van and what it was used for.
You also need to remember the VAT. If the van was bought as a business tool then they will likely have claimed back the VAT which means you will need to pay it when purchasing the van. If it was bought privately or the vat was never claimed back then you won’t.
Try not to get too hung up on ‘+vat’ just compare the final price including vat with other vans - the total price is what matters and a van advertised with vat included will be in the same ball park as one that has its price listed as +vat.
For spec I would suggest Highline as it comes with ac plus several other nice to haves like heated windscreen, cruise control, multi function steering wheel, heated electric door mirrors, reversing sensors, colour coded bumpers and mirrors, multi function computer and more. Startline is the commercial spec with plastic bumpers and Trendline is in between.
Here are a couple of snaps of my previous van and current van
Good luck and keep us updated.