@Salty Spuds is right about Ile d'Oleron - we much prefer it to Ile de Ré or Noirmoutier (with Ré at the bottom off the list!) but, as he said, it's not a particularly good base for day trips out unless you're nearish to the bridge. He's also correct about Camping Les Saumonards which is a great campsite that we've been to several times over the last few years - very close to the beach, in a lovely forest location with loads of walking / cycle tracks, lots of wildlife to be seen and only about a 20 minute walk into a small town (Boyardville) with a few small shops for grub and plenty of eating establishments. The downside is that if you're there, you're already 30 - 40 minutes away from the mainland.
As to turning up on spec, we never book anywhere but our rule is the we always try to turn up at a site before 1600 hrs and have usually found that by the time we're set up with a cuppa in hand, others are flooding in and at some of the more popular sites getting turned away. That said, I doubt if anywhere would be too difficult to get in at the time you're planning on going indeed, you're main problem if you're there for October will be sites closing down for the season. On that note, IMHO you will also need to get very lucky with the weather if you're ambitions run to dry and early 20s, at least consistently
Also, don't worry about the driving in europe thing. You soon get used to it, the roads are better and IMHO driving standards better as well. We always use Waze of Google Maps for navigation and their ETAs / timings usually prove fairly accurate. I personally wouldn't fancy doing Dieppe to the La Rochelle area in a day (but neither of us particularly like long drives unless we can avoid it) - we usually stop about midway in consultation with the ACSI guide.
On the subject of ACSI, we wouldn't be without it and you can sometimes get your investment back in a couple of night's pitch fees. I'm quite a techy person but I'm also maybe a bit weird in that I still find physical reference books quite nice to use. That said, I also have the ACSI apps on my phone and iPad.
Out of interest, why the ferry from Newhaven? Each to their own but we prefer the tunnel - there's a couple of campsites very close to the UK terminal which allow an early start and you can then make decent progress on day 1 in France, Either that or we get a mid-afternoon train and then stay at a site close to the other side allowing an early start on day 2. Again, it's a personal thing but when on a road trip, there are few things worse that having to find somewhere to kip when it's late, dark and you're possibly knackered from a day on the road.