In order of precedence:
- Cam belt
- Ball joint boot
- Shock absorber
No. 1 Priority: I'm not sure if the Peugeot has an interference motor, but will assume it does since most vehicles these days are built as interference motors. An interference motor is one where the pistons and valve train occupy the same space inside the engine, but at different times. The cam belt and cam are what achieves this happy dance. If the cam belt breaks, you run the risk of having to replace the entire engine. Reason for number one status: if it breaks, it will cost a huge amount to get your car fixed.
No. 2 Priority: The ball joint is important. Depending on the state of the ball joint, this could be an expensive or cheap fix. If the boot is completely destroyed and you can tell there is major contamination, you'll probably need to change out the entire ball joint, which in and of itself is not overly expensive, but labor costs are going to cause the cost to go up dramatically, as well as requiring a front end alignment. On the other hand, if it's just a cracked boot which needs to be replaced, you can replace it with a new one relatively cheap. Most people would not be able to perform either of these replacements at home, so labor costs will bring up the price somewhat. I suspect if the boot had been really bad, it would have failed MOT, so would bet the ball joint is still in good shape. Reason for number two status: if you wait too long to get the boot replaced, it could entail replacing the entire ball joint, thus cost more over the long haul.
No. 3 Priority: The shock absorber helps with vehicle handling and stopping. Since this is on the rear of the vehicle, it is not nearly as prevalent issue as it would be on the front of the vehicle. You'll find though, when replacing these, it's not nearly as expensive as you'd think. If you are at all mechanically inclined, with a set of basic hand tools, you could replace these. Replace both when you do. Replacing these becomes important as over time when all of the oil runs out and it can no longer dampen the suspension, it will wear your spring out on that corner of the vehicle, which will incur a larger cost to fix. Reason for number three status: non-repair over time will cause a greater cost to fix, but for right now is not an emergency.