I only know VW Transporters, but know them fairly well.
Huge info source here
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'm assuming you're looking exclusively at T4's and that T5's would be over budget).
I wouldn't attach too much importance to mileage or year - but much more to condition and a *verifiable* service record. On my most recent buy, I saw more clocked vehicles and probably fake service histories than I ever have before).
Well looked after T4's do terrific mileages and offer superb longevity. One key difference is the older 1.9TD and 2.4D models versus the newer 2.5TDI. Very marked difference in power output. The older ones are slow-coaches - the newer ones brisk - with a price premium of course. Other than that, few significant changes over the model years.
Cambelt change interval is now recommended to be 60000 miles (decreased form the originally recommended 80000 miles), OR every 4 years regardless of mileage. You may find some do light mileage only but are way past their due change at least every 4 years. =A3500 for an official dealer to change. Look out for others that skimp and don't change the tensioner/idler/water-pump at the same time - which the dealer will be doing.
Any official part is eye-wateringly expensive. Several good aftermarket suppliers. GSF is probably best of them
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prices are a bit inflated as T4's are a very popular base for surf-van conversions.
Personally I won't look at anything that's been customised - very, very rare that it's done to a high engineering standard.