Depending on when you're planning to start, bear in mind that Gardiners are planning to restart work on their super zero systems in spring. They are crash tested and will be a damned sight less than the price you've been quoted. I can't see how Ionics justify the premium, but as long as people pay it, they'll charge it.
When people tell me they have load straps holding their tank back, bear this in mind. When Gardiners first built what they thought was a well designed crash-proof system, it came straight through the bulkhead. The video was horrific. Now, that was their first go, based on good engineering advice and it still failed miserably so they completely redesigned it. Consider the potential for the failure of a system build on the back of another window cleaner saying "straps will be fine". You won't get a second chance to redesign it.
Finally it seems to me that the economics are simple. If you're going to end up using the system for the next eight years on your van, the cost is pretty irrelevant. If you start up and give up in three months, it'll hurt you for a long time. Buying it as a cold start is ambitious, but if you have the drive to make your business succeed, it might work out.
I should stress that I'm in no way a fan of Ionics (for many reasons).
Vin