If this goes wrong, then you have a reasonably serious problem. The car will stall regularly, whist idling, and the tick over will be irregular, and bumpy. On top of that, your fuel consumption will increase, and your emissions will increase. You'll probably find that the mix is burning a bit rich. Unfortunately, there is no easy cheap solution. You can do one of two things. The first is replace the part, which is around GBP 400 (or GBP 120 if you buy it reconditioned), but in the long run it's definately worth it. Secondly, you could get someone to tune the engine to a compromise position, where emissions are not as low as they should be, and that the engine runs slightly rich to avoid stalling. It will still stall, especially in the cold and the damp, but the problem is lessened. Note, though, that this is only a temporary solution, and you probably wouldn't pass an MOT with it.
On the Coil, on the right hand side of the engine as you look at it from the front, you will see not only your HT leads, but also four smaller wires, connected to the top. Being small, and not that firmly attached, they are prone to rusting, and losing contact. If this happens, then car will stall. The revs will drop sharply, several times, and then die out all together. It sounds pretty serious if it happens to you, and I even got my breakdown service out, because I didn't know what was going on. But the solution is to start up the engine again, and move the connections, until you discover the loose one, and then simply clean the contact, and ensure that the connection is sound, and then everything should be OK - at least, for a while!
Fuel Injectors are fairly delicate pieces of equipment. The needles that they inject fuel through are by no means large, and any kind of dirt in the fuel line can be disasterous, resulting in a total inability to start the car. So if you have a fuel flow problem, and dirt get pulled through, then they need to be thouroughly cleaned out. To get the car started again, usually changing the fuel filter will do the trick. What you need to be sure of is that if you change the fuel filter then you buy the right one. Generally, the Peugoet 205 GTi filter is the one that you want, which will cost you around GBP 10. Also, it is worth putting injector cleaner in the fuel once every six months or so, to keep them running smoothly. Don't run the car low on fuel, either. As a car gets older, it collects a lot of gunk in the bottom of the fuel tank, and if you start sucking that into the engine, then you are going to risk a blockage in the injectors. If the worst comes to the worst, then you can unbolt the metal bar that runs across the intake manifold, and remove the fuel injectors from the engine, and clean the holes in the engine, and around the nozzles of the unit. But be extremely careful not to allow rubbish to fall into the cylinders whilst they are exposed.
Because the GTi has central locking, the locks are quite expensive (about GBP 50 for one servo). If they stop responding to the signal from the central locking loom, then you don't have a real problem, you just have the annoyance of having to lock all the doors yourself with the key, or after the central locking has been put on. If, on the other hand, the lock seizes, then you won't be able to lock or unlock the door. In that case, to at least be able to open and shut the door, remove the door trim, and unbolt the servo, and unhook it from the lock, and hey-presto, you can operate the lock manually again.
Remember, alloy wheels may look nice (and I have to say that the Citroën Visa/AX alloys are particularly good, without being biased), but they are softer, and more prone to impact damage than steel wheels, so if you subject them to an impact, then you're more likely to do expensive damage. If you discover that any of your tyres are leaking slowly, and it's not due to a puncture, then you should check out the possibility that you've damaged the rim of the wheel, and thus caused the tyre to fail to sit properly. This may involve a new wheel, but a garage will be able to tell you if they can do anything about it. If you're suffering from surface wear on the tyre, and the laquer is wearing off, then it can be quite good to have them re-dipped, at a cost of about GBP 30 per wheel.
It has been noted that the speedometer on the Visa GTi is particularly tempramental. This is because unlike most conventional speedometers, this one is electric, and prone to malfunctions. These are most often manifested as periods where it simply doesn't respond, and other cases where it jitters when the car is stationary with the engine running. It can also mean that it's accuracy at measuring the speed of the car can be slightly out. If you have these problems, changing the speedo cable will have no effect. The only real solution seems to be to buy a new one.