I don't really know how the used market goes in the UK...although I did spend a week in London shopping for a used bike....first time ever to London. I found a beautiful Triumph Tiger.
In the USA used prices kind of level out at some point...on old DR's of the vintage you describe, that number would be at around $1200 to $1500 (roughly 600 to 750 ukp). So even old boilers tend to hold some value....too much IMO.
The real problem is...most of these very old bikes (at least here) are JUNK! Not Africa material. On the other hand you can find DR650's only a couple years old for $2500. I'm talking a 2004, 2005 with very low miles, usually under 10K miles.
So save up and go with a newer, low mileage example. You'll be glad you did.
By the time you put a 15 year old bike right you may have spent more than its worth. If you wing it an just go for it on an unknown quanity....well make sure
you can trust your friends to hook you up with local transport to get home.
The other important thing is the major re-model Suzuki did in 1996. Many important improvements both in the engine, suspension, chassis and brakes.
Two American desert racers...factory racers for Suzuki....went to Japan and helped design the new bike, using local test circuits and working hand in hand with Suzuki engineers.
They then brought the Proto-type bike to the California desert and rode it for a couple months, then went back to Japan for changes and more improvements. In a year, they had a "new" DR650. The DR650 SE.
Not much has changed since.
If you're mechanical and can suss a bike out yourself, maybe you can get an old bike cheap that'll do 20K miles. It would be good if ou can fix it along the way....you may need to?