I would say Edd was being a little generous, £150 balance should be £20 a week, £90 balance at least a tenner. £25 is a bit harsh IMO, but terms are up to the individual dealer ultimately.
As for the finance, a 3 year deal is going to cost you 9% each year, so with the dreaded VAT as well, you are going to pay over £4k. But with Snap-on Finance you can always get a refund of interest if you settle early. So if your credit card company offers you a 0% deal, for example, you can use that to pay off the account and you're quids in.