
I said that I had a cheapish back box fitted recently and I think they've cottoned on to the fact that the HDi needs very little 'silencing' BUT it leaves you with a horrible deep grumble, sounds crap.
I'm only saying this as most powerflow types are basically a straight through silencer with parallel sound absorbing glass wool so I think you'd be left with the same old grumble.
If it was me I'd want something mid range from an exhaust place - in fact I'd be ringing national tyres or someone and asking for cheap, mid, and high end prices to choose from (most people settle for the cheapest so they don't tend to stock the better ones).
I'd get the best I could afford and then I would paint all the exposed bits that rust in my own time at home.