New wheel advice for *UK shaped* hiace - help

Hello everyone,

I have a UK shaped 2005 hiace work van which I am kind of converting into a weekend budget camper.  It urgently needs new tyres and as I was thinking about cleaning up the rusty wheels I thought this would be a good opportunity to upgrade the wheels one time for a different size if possible?

I am new to this site and to doing any kind of modification to a van, I don't know much about wheels but I have read a few posts that I don't really understand much of it!  It seems that none of the posts were aimed at UK shaped hiace sloping front vans (which i think is the same as a Granvia?)

I know that there are issues with wheel strength and comfort and although I am rarely carrying much weight, safety is very important.  Also I am not necessarily after expensive uncomfortable low pros, and I am considering putting on either steel or alloy ... but my main query is regarding the overall size, as I do think something slightly bigger overall would look much better.

On the door sticker it says:
195/70R15C (which is what is says on the tyre too)
But on the door it also says:
104/102S

(I would include pictures but I am not sure how?)

Does anyone know what the biggest wheel I can install is without much (or any) expensive modification to the van?  Looking down the side of the van it appears that the wheels have room to be wider, or to move outwards slightly without sticking out of the arches.  I am not sure if that is an option but thought it could help with clearance perhaps?  Turning the wheel to full lock, I think that there is room for the wheels to be a couple inches bigger in diameter but I don't want to buy something and it to be wrong.  

Any help (in layman's terms) would be much appreciated!

Also does anyone have any images of this shaped van modified? There isn't much inspiration out there, just VW's in the UK and flat fronted hiace's online.

Cheers.