A competent person can fit a tank that will withstand the forces of a crash as well as any professionally fitted system,.. it takes a lot of thought & a good bit of effort, but it is possible.
That said, I've seen plenty of DIY setups installed installed by complete incompetents,....
ALWAYS inform your insurance,.. it doesn't matter if you consider it a modification or not, it only matters how they consider it.
As for hot,.. an immersion element is by far the cheapest & safest DIY job IMO. I have a fogwash personally, but I've gone to the trouble of installing a flue and multiple vents in the van because of it. Again, its essential to inform your insurance!
I personally like the look of the pure freedom hot, but can't justify the price when compared to DIY.