H'm, I'm a bit surprised the Bromptons are out of your price range. They are a bit expensive in the US but the lower-cost models are around $700, and I assumed at least part of that is due to taxes / tarrifs and low volume of imports.

The "old" Vitesse D7 had a derailluer (like the Speed D7 does), the "new" Vitesse D3's and D5's have internal hubs. So basically, the transmissions are a little different, not sure what else. One thing about Dahon is that they change their model line-up fairly frequently, and they use a lot of non-standard parts, so on some occasions it can be difficult to get replacement parts on the older bikes. For example, they changed the handlepost designs in 2005, so if that "old" Vitesse was made in 2004 and the handlepost ever breaks, you will have some issues with that repair.

If you pick up the "old" Vitesse you might be fine, but you're already at a bit of a disadvantage. I'd check with the sales person to find out if it's new or used, and what year it was made. If it was made in 2004 or earlier, I'd haggle them on the price.

Contact Downtube directly to find out about UK sales.