Haha! But the L series
is a good engine... so good they added another cylinder and called it a TD5.
Basically, if you reverse at very slow pace and push the clutch down, the car should slow but not like it's got a handbrake on. If the car feels 'tight' like the handbrake is on, then chances are the VCU is shot. It's a quick simple test, but also they get hot after 15 miles or so if they are shot too, and of course there's Hippo's weight test.
If you find a good L series then I think you'd be happy with it. Best thing is to drive one and a TD4 then look for the 'normal' faults with both on the internet then decide. I agree the TD4 is a better engine when it's running right, but IMO they are more temperamental as well. TD4 with Ronbox is a great thing as long as everything is working. My TD4 had 11k miles and all the usual things had gone wrong with it needing 4 new injectors, all tensioner pulleys, MAF sensor, intercooler hoses, thermostat, and it had had a new clutch/dual mass flywheel and clutch hydraulics at 84k.
My L series had 150k and the only thing I did to the engine was change the fuel pump belt (simple job) and everything else was original. It's a bit more 'vocal' than the TD4 and not as powerful, but even they can be tuned. The TD4 drives like a petrol.