Guessing, off the top of my head:
LPG kit complete with tank
It's condition is unknown in that it's never worked properly, so peanuts I'm afraid 
full heated leather
In it's condition, and being facelift - £200
various body panels
Not a lot - bumpers maybe £50 if good. Headlights, etc.
headlight washers
Are they not built into the bumpers on facelifts?
3.0v6 engine and autobox with 76k
AR35 - £80. Engine - £250 on ebay perhaps but you have the hassle of taking it out
very tidy well looked after elite alloys on recent tyres
£200?
irmscher grille
Not a clue - £100??
bose sound system inc sat nav
If it's CID like mine with the headunit and entire system - £300 maybe?
recent exhaust - worthless
It's certainly worth thinking about, in your situation!
If you do break it, I WILL have the LPG kit off you 
Nice one James.

Brilliant way to get cheap parts for your car.
Will have to remember that method.
Shoot it down 1st and offer peanuts later.
Especially when yours needs a new exhaust and the LPG is probably something or nothing. :-/

There is a fundemental flaw in your theory.
Danny's car is a Saloon, mine is an Estate. The entire cat back exhaust system is totally different.
So how can this be the case?
In addition to this - why on earth would I travel a 500 miles round trip (which would cost me about £80 or £90 in fuel) to buy a used pattern exhaust (which wouldn't fit anyway!), when I could click a couple of buttons on ebay, and get it delivered to my front door!
As far as the LPG is concerned, let's look at the facts. It's never worked properly and despite spending a day (voluntarily, I may add) trying to get it to work for Danny, I couldn't fix it. With kits that we know well going for very low money - it's common sense that one with an unknown issue, that requires a good day removing neatly from a car - isn't going to hold a high value.
I've spend countless hours helping members here and TBH at the moment, I feel that it's ME being shot down, and I really don't see why.