I was only able to sort out what I needed thanks to playmoDB as I didn't have any plans at the time as this was just before I spotted the 4087 set on eBay (see here: http://www.playmofriends.com/forum/index.php?topic=5361.

and, in fact, I missed one item, a System-X green connector which I will buy on my next order.
The exercise proved two things - it is possible to buy a complete item from DS but it is expensive. This THW 4x4 cost me more in parts than the 3181 set does, which is basically the same vehicle in fire brigade trim and with a figure and accessories (here:
http://www.collectobil.com/catalogue/items/3181.htm). However, if you want an item and the only way is through DS you can do it, and the extra cost isn't enormous, about £3 extra in this case. As I wanted a THW 4x4 by itself in any case as an extra vehicle without the boat and trailer, it was a worthwhile experiment.