Yeah, I know. However, it's a question that only you can answer.
45bhp 451 CDI
There are 3 problems with this model.
It's older than the 54bhp model,
It's slower than the 54bhp model with a slightly different turbo setup,
They are generally the same price or more expensive than the 54bhp model.
The good thing about this model is they are far more reliable (although mine did die at 60,000 miles).
Expect in excess of 250,000 miles with the diesel engine.
The price over the other model is because the 54bhp version has a bad reputation for reliability.
54bhp 451 CDI
There are 5 problems with this model.
The EGR coolers eventually leak and drain all your coolant,
The turbo and DPF temperature sensors are prone to failing,
The DPF pressure sensor is prone to failing,
It's slightly worse on fuel than the 45bhp model,
The DPF can get clogged up if you don't drive it properly.
The good thing about this model is they can be cheaper to buy and far easier to find for sale.
Fill it with diesel and drive. Don't worry about remaps, just leave it alone and it'll be reliable.
Do your normal monthly oil and coolant checks.
Fill it with diesel and primarily drive medium to long distances.
Don't do regular short journeys as the car won't get hot enough to clear the DPF.
Always have a spare few hundred pounds just in case one of the sensors fails.
Don't buy a 54bhp 451 CDI unless you drive over 50mph for at least 10 minutes for 3 out of 5 of your journeys.
Don't buy a diesel 451, not even the 45bhp version. Slow, stop/start traffic and short journeys
just won't bring you particularly good MPG. Go and buy a small petrol car like the 451 turbo.
Don't buy the non turbo MHD as that is the least reliable engined smart available.
If you drive the petrol 451 turbo carefully, you can get better MPG than the MHD anyway.
If you drive the petrol 451 turbo fast, it's way more fun than the MHD and the diesel.