I had a BRC system on my previous car - a Subaru Legacy 2.2. Superb system, can't fault it at all. No real difference in performance between petrol and LPG - but that's dependent on the type of engine it's fitted to and the limits to which the LPG ECU can change the way the petrol ECU runs the engine (ie advancing the ignition timing etc). Could take a little while (3 minutes) to switch over to LPG when in the depths of winter, though that's probably more down to the Subaru engine not getting warm enough than anything to do with the BRC kit.
You may or may not need Flashlube (depends on your engine, the Z18XE does not need it), in which case a decent installer will advise you on that.
I agree with the previous comment - choose a good installer, rather than a system. If they fit it well, it'll be trouble free for the lifetime of the car. If they fit it badly or don't know how to tune it for that type of engine, it'll be troublesome.
And yeah, think about where you want the filler mounted. I think high up next to the petrol filler is best (which is where Vauxhall put it), rather than low down, as it saves you grovelling around in oily slush in winter. However LPG pump hoses do tend to be shorter than petrol ones (because they're much bulkier) so if the place you intend to fill up most often doesn't have two LPG pumps (like most Asda ones), you might want to consider something more central.
Don't forget that the Vectra C was available from new as an LPG version (Dual Fuel 1.8), so it's very suited to it. Certainly my 2004 1.8 Elegance Dual Fuel is the best LPG car I've driven.