It's easy to get disheartened when small issues start to pile up and make the car 'feel' worse than it is.
Your initial list reads to me as:
1) Niggly misfire you've spent time chasing, which is now really annoying and the primary thing to get fixed
2) A coolant leak over weeks which you're now worried is a head gasket and would mean it's maybe not worth fixing but its much more likely an elderly hose, reservoir cap or anything else given the leakage rate. You could most likely ignore this for months or years
3) A rust patch on one sill, which on a 196k clio is pretty light really, most will need arches, sills or jacking points even if they don't look like it yet and repairs are fairly simple.
4) Other minor issues which add to the pile but are of no real consequence and probably won't bother you once you fix the big ones.
So the real question is what's causing the misfire and how quickly that can be fixed to put the smile back on your face and motivate tackling the other things!
I had a similar sounding situation with an intermittent starting fault, which after all but declaring the car scrap, turned out to be a failing relay.