+1 for the twin axle. I wouldn't have a single axle caravan - too much side area, and too hard a ride for the contents too.
IME, and we have been caravanning for donkeys years, a good 4 berth twin axle caravan needn't weight more than 16-1800 Kg - which, despite the large frontal area, a 300 TDi 110 should tow with consumate ease. It might be slow up steep hills, but on the roughly level, it'll cruise happily.
Worth making sure the A frame bushes ( all of them ) are up to scratch, and the rest of the rear suspension too. The better the standard of general servicing, the better the experience will be - but that really goes without saying....Whilst not necessary, you could also consider a boost pin, and if you can be bothered a full width intercooler... and an auto conversion or a clutch servo... and.... and

we tow a 1600Kg Abbey vogue2 610 twin axle with either D1 300Tdi - one is a tweaked (de-electroniced and higher power ) auto, the other a relatively standard manual. makes a nice stable rig, and with your longer wheelbase, it will only be betterer