Diesel RIBs do come up for sale second hand quite regularly, but as you've seen they don't stay for sale for long!
Under 20k you are right to think that you are likely to be looking at an old Halmatic. They tend to be very tired looking when they are released from the MOD, but are tough enough to be brought back to a reasonable standard -- don't discount the possibility of retubing and old RIB, it may still come within your budget.
The other possibility would be one of the six metre RIBs that have been fitted with the Volvo Penta 105 hp diesel. I've only seen a few (Ribtec and Coastline spring to mind), but this would give you a much more modern RIB and engine, although not as bulletproof as a Halmatic.
Either way it won't be a particularly swift boat, but you've already taken that into account. I would guess at about 25 knots for an old Pacific and around 30 for a 6m boat with the little Volvo.