What do you need it to do?

I think the first thing you have to decide is what do you want the vehicle for. We had certain criteria which the vehicle we wanted to end up with had to fit. On looking round quite a few, without paying serious money for a purpose built piece of kit, we concluded the only affordable way to get what we wanted was to do it ourselves.
Being from Yorkshire and having sometimes been accused of making a squeaking sound when I walk, it still grieved me to have a dedicated vehicle, of which we may not even enjoy using as we'd never tried the motorhome way of life, stood around when it wasn't being used. So my criteria was 1/.…As I have a motorcycle, it would be great to be able to take both to the likes of the TT. races for example. 2/ It needed also to be able to take more than just two people on the trip if required. 3/If I needed a trip to the tip,builders merchants or to even pick up a new fridge or lawn mower, always having had a van at my disposal would tee me off so much if I had one I couldn't use.
So, these become very major factors that govern your layout, so play around with ideas and once you have some fixed ones, put them to one side and start a second time with a clean sheet as sometimes, even the third idea may be the one you go for wondering why didn't you think of it before?
I then searched for a Yorkshire (FREE) drawing/cad program on the internet and found lots, finished up using "Sketch-up". This allows you, after a night or two's practice, to create 3d models to scale which you can rotate, look from different angles and even go inside and look out. Also,if you go on the public domain part of the site, there is even a Sprinter 3d model you can download of which I was able to fit my layout inside of it.
I then did a mock up with bits of wood, worktop, cardboard and mocked up the whole thing to make sure there was room to move about without knocking lots of skin off. The photo shows the second mock up but as I said, that wasn't the one we went for either but may give you some idea. Spend the time at this stage, it's worth it!