Do you need the van for work, or will a car do is the first question. If a car will do, then almost certainly a co car will be the 'best' option - if not the outright cheapest - even more attractive if you not a higher rate tax payer.

Should be pretty easy to work out if you can find out what car you can get; look up its list price and BIK rate (note that BIK rates are going up quite a lot in the next few years, so you need to forecast that). This will give you an annual cost. Now add up tax, MOT insurance, servicing, tyres etc for a run about - then add at least a couple of grand (1k cars are, in my experience, near scrap unless you are lucky) to buy it, and you will have 2 numbers to consider.

Look up any car review/makers site & they will give you the List price for that model & engine size & they will tell you the BIK annual cost for a 20% & 40% tax payer.

Divide that by 12 and you will know how much you will pay extra in tax per month - easy!