If I remember correctly, the TDi motor put out some 81 or 82kW, nothing to write home about but the old motor had good low down torque characteristics, purposely suited to slower off road slogging, which by extension includes towing.
Even solo, I think it's top speed was some 139kmh, which is irrelevant in this type of application. I recommend a maximum towing speed of 100kmh which I am sure the TDi will manage once up to speed.
Sam has the slightly more powerful and modern TD5 motor which in Defender form has 90kW(300Nm) and in Discovery 102kW(340Nm), but he is towing a much heavier van (Xplorer).
Stability and towing dynamics will be good, with frontal resistance of the caravan offset by the already bulky Defender profile. I don't think it would be fair to compare the Defender with a Telstar except to say the Defender will be a vast improvement as a towcar.
I can't tell you what fuel consumption you will achieve, but driven conservatively, I would estimate that you could manage between 5 & 6 km per litre (approx.16,6l/100km)