Hi,
I upgraded my wheels/tyres a few months ago. Using this tool
Tyre Size Calculator I can see that your proposed wheel diameter will be 8.9% bigger than stock:
I went for 16x7 rims with 205/55/R16 tyres. My diameter increased by 5.5% from stock. There is still some room in the wheel arch, so the extra 1/2inch of radius to go up to your proposed tyre size from mine might be ok, but I'm certainly not qualified to tell you. I can take pictures if you like? let me know. The 'front' of the rear wheel arch is a little tight when unloaded on mine.
I lost some acceleration upgrading, you're set to lose more, which you will definitely notice, especially in hilly areas. My speedo with stock wheels was over-reading by about 5% so now it is perfect - as Gerry mentions above even bigger tyres still will make the speedo under-read even if you had the same situation as me.
In terms of safety, again I'm no mechanic, but your extra tyre width and contact area should enhance braking/traction. Folks warn of increased wear on wheel bearings etc, not sure at what point this becomes a serious concern.
A friend went for 205/60/R15 in his NV and they look rad - the tyre wall bulges out more than mine and look a bit more 'off road'.
I can see why u wanna change - the stock wheels/tyres look tiny and ridiculous with the big empty wheel arch. Changing mine out completely changed the look of the van. Now looks meaty, handles better, brakes faster (especially in the wet), tracks straighter on the motorway.