My tuppenceworth for what its worth, is that safety is top priority. For that reason I would never ever drive around in a van with a DIY tank. A fella on this thread said the tank was secured to the floor of the van. That's totally unsafe in my view, the van floor has no strength at all, and bolts will just tear through it. Needs to be fastened to the chassis. BUT I'm not bothered enough to argue about that with anyone, after all it's up to each of us what we decide to do and what's important.
Aside safety, I don't think a pro system would MUCH more that a DIY wouldn't do, maybe a bit more convenient for changing resin, a neater installation etc, but really that's down to personal choice.
I didn't have the money to spend on a top-of-range ionic system, but ionics Pure2o systems (which is a sort of value range) are very well priced and solidly built IMO, so I bought one of those for what I consider a perfectly reasonable price, which (suprisingly to me) included fitting. (for anyone interested, I think it was about £2300 all in, 400 litre system)
Although I am quite good with fixing stuff like this, I have to be honest and say that having the fitting done for me was a great boon. It was done in less than a day, and to a much neater standard than I would have been able to myself, and I'd even go so far as to say that it looks better and neater than any of the DIY systems I've seen pics of on here. Added to that the fact that it has never let me down, ever, not once. I've never lost a single day's work to something breaking or going wrong. Reliability is worth paying for IMO.
Apart from the cost, one good reason for going DIY is that if a problem occurs, we're in a better position to fault find and sort it out quickly without having to rely on a third party to do it for us.
I suppose that's true, as it goes, but it took me about 5 minutes of staring at my Pure2o system to understand exactly how it worked and what each bit did, it's not complicated. As it happens, nothing has ever gone wrong with my system but if it did I wouldn't be stressed about finding and fixing what it was. I would always advise keeping some vital components as spares (eg a pump, fuses etc).
If I was advising someone now I would say yes you could build your own DIY system, and it would certainly save you some money. But honestly, you have to rely on the system to earn your living, and the extra money spent on a professionally built and properly installed system is probably money well spent.