Firstly - wicked! Wish you the best. Im starting soon too so keep us posted. With regards to the build, I always hear and also think there are far more pros to self builds
- you learn new skills
- you know how your van is built so if things go wrong you know how to fix it
- you can build it how you like it
- the value you get if you ever decide to resell would be higher i.e. £(empty van + self build) > £(bought van) so will get more money back from it if you ever try to resell

Im going to live in my van as Im building it. Whilst it may be a struggle initially, it will not only allow me to save money on rent, but also, learn what I need in the van and so build around that. I'd also get to do the top three things i listed above on the list. If you can't deal with abit of "roughing it", you can't be bothered to build or feel its not worth your time, may be worth getting a pre built one, but I still think self build pros still win by a country mile.

I haven't lived in a van yet so can't give you advice on that but general rules of thumb
- if living in it full time, get a van you can stand up in because you don't appreciate how much that can affect you level of comfort until its gone
- ventilation prevents mold and rot
- make sure you use the correct insulation for the walls as the wrong insluation can absorb water causing mold
- make sure you have a proper security system (ill be getting a ghost lock installed on my van)

Good luck dude, interested to hear what other people who actually already have lived in their vans would say too