It was supposed to be a little tongue in cheek! But it gives you some idea of how complicated they make these things now.
I worked in a bodyshop in 93/4, I was allowed to work on simple cars like Metros, Mk1 Astras and Sierras, but I was kept away from Mk3 Cavaliers, Mk1 Mondeos and the like for a long time because they were considered to need specialist, experienced attention, and because the build tolerances were so much tighter that a minor error would become glaringly obvious when it was all put back together.
A bodyshop should be able to repair it unless you've really done it well, you'll be surprised at just how much you can hide with a bit of filler!

Pal of mine bought an ex rental fleet (Hertz, IIRC) Omega at 14,000 miles. He's now three years on, 40,000 or so miles further on and has never had a problem at any time, despite the car only having had one service while in his hands.

My wife has a 9 year old Peugeot 106 diesel with the 1.4 litre aluminium engine with steel liners. We owned it from new and after warranty I have serviced it.
I use a 50% mix of Halfords coolant conditioner and change every 5 years or when I have to drain the system (to replace a hose). No engine problems at all (touch wood).

It has only covered 36k miles (!) and no gasket/head problems but my sister in law had one and after 4 years she had head gasket problems.. but then it was garage maintained and driven hard and unloved.

If the system is drained and refilled with the original coolant you can end uo with the worst of all worlds - weakend mixture and extra oxygen in the system. Also systems have to be bled to get rid of airlocks..now what chance that the average garage in a hurry will part fill the system, bleed it, fill again, bleed it, squueeze hoses to expel air, run it, bleed it again... Well that's what you should do to make sure there is no trapped air.

Trapped air = localised hot spots = failures in the future.

Given the incompetence of most garages and their total lack of quality control (they should monitor complaints by mechanic and model.. most don't) chances are none of the above happens.

People have been adding LPG to diesels for a number of years now, especially in US tractor pull type events. The gas acts as a catalyst, helping the diesel to burn more effectively, thereby creating more power. The limit to the power is the exhaust gas temperature, which should be kept below 1200 degF, which is the melting point of pistons/rings. Ideally it should be kept to below 1000 degF. Unlike petrol engines, diesels can't be run too lean, so the more you can get in there, the better. If you increase the size of the intercooler, you can bring down the EGT, on a roughly 1 deg for 1 deg basis.

How does it know how much you filled up with or if you have done at all? It'll only have a fuel flow meter on the tank outlet side, and (as many have shown) isn't generally that accurate.

My car has the same set up as your Citroen. I must admit it would be great if the car did have extra sensors that could determine the fuel economy between fill-ups....