Hi cipsony. Welcome to the forum!
I can only help you a bit, as I'm in Canada and our cars are somewhat different from the Euro variant. But we have a lot of information on this forum, with plenty of members in that region as well.
1. There are a few changes that I am aware of in the 2017 models - mainly more available options and an additional colour (yellow). The 2014 European cars had the "spider eye" headlights, while I think the covered ones are now standard. I've heard that there are some changes to the tub for the 2017 coupes (making it the same as in the Spider, slightly heavier, but someone may correct me on that.
2. I don't know.
3. Another member put this picture up today, of his 4C on the track with the standard suspension:
The race suspension typically does not roll anywhere near that much by that far through a corner.
Someone else from the UK, who has had the opportunity to drive both, noted that hard driving (on a race track) is the only place where you will note the difference.
The standard suspension puts the car a few millimetres higher than the race one. But you'll likely never notice that.
4. The service schedule requires a bolt tightening at 1 year, and every 2 years thereafter (or 20,000KM / every 40,000 thereafter). Be sure that was done, or have it a condition of sale to inspect and re-torque the bolts. It is not cheap, and there is potential damage to be done. I'd have a trained Alfa shop look at the car anyway, as the tub is a unique item that mere mortals like us are not able to evaluate. It is pretty robust, but if broken it is almost certainly a write-off. I don't mean to get you worried, but better safe than sorry. Make sure that the updates (flashes to ECU and transmission control unit) are done or get done after you own it, and that all the Alfa "pit stop" maintenance was or gets done. There have been only a few issues with the car - some leaking or condensation causing water in the cabin and trunk, and some transmission problems (mainly fixed with software, but some more serious repairs were done). Be sure that the panels all fit well, and give the car a good car wash to ensure no trunk leaks.
5. The CF side vent was put on to solve transmission heating issues with US cars (which are heavier than in Europe - as are the drivers
and there is perhaps more stop-and-go traffic in large American cities). Stop and go traffic heats the clutches. The vent simply directs air to the transaxle. Of course, at slow speed, there is no air flow. At higher (highway and track) speeds, there is plenty of cooling already. So I'm not thinking that this option on Euro 4C's is really worth it. We have a lot of European members who's cars have seen significant track miles without this vent, and without issue. If you get stuck in stop-go traffic, do not let the car crawl in first gear. Either apply the brake (hard), or the gas. idling off seems to be hard on the clutches. Put it in Neutral for longer stops (trains).
Good luck in your shopping! Be sure to let us know how you make out!