I've got a L series and I use it for towing a 22ft caravan. I don't go above 60 MPH as it ain't that stable at higher speeds. I've got a TUNIT module although I don't honestly know how much it improves the thing as it was fitted when I bought the car and I've never tried removing it.
In general I find the car has plenty of power for the job, providing you remember that pulling a caravan is like driving a lorry. Forget high acceleration and high speed. No point is thrashing it as there is only one gauge that moves quickly and thats the one with the fuel can on it. In addition when planning a stop I always throttle back a couple of miles before so to let the engine cool a bit. When I first started towing with the FL1, I pulled into a services, parked up and switched off (ran the turbo down as normal). Then noticed the hot oil smell. When I left half an hour later I was chomping up the slip road when the engine totally lost power for a couple of seconds. Cacked meself I can tell you as I was merging with the traffic at the time. Fortunately it recovered and all was well. I believe I had lost fuel pressure. I know with petrol cars that it is possible the vapour the fuel in the fuel line and this loses the pump priming, hence killing the engine. Don't know if this is possible with a diesel but it certainly felt like it.
I suspect the TD4 is faster but not by a great amount. Also the L series is proving itself to be a tough little lump that should serve you well. the TD4 doesn't seem to fair so well in that respect..