There are thousands of 204 T6's running around in perfect health, (nobody creates a thread to tell the rest of us that he's had another year of perfectly trouble free use from his 204) but I've seen an increasing number of threads on here about 204's with catastrophic engine problems, some with low miles and good service history. It's not that you're likely to buy a wrong'un, it's the fact that if you are unlucky it's disastrously expensive!
I'd be looking for a 150 instead, more than capable of keeping up with traffic even fully loaded and with less likelihood of issues, although every vehicle purchase involves an element of risk.
And in answer to your second question, I don't think there is a great deal you can do, apart from the normal stuff, to mitigate buying a bad van unless you can have a 1,000 mile test drive before purchase to see how much oil it's using.
I think the easiest and cheapest thing to do with a van that's starting to use more oil is to part exchange it or sell it, which is a deal you don't want to be on the wrong end of.
Just my opinion, of course!
