23mm tyres are fine. But 25s would be better with a touch less pressure in them. I usually ride Vittoria Open Pavé (which are 24mm). Normal road bike is fine. A couple of the longer cobbled sections can be hard work - big ring and don't grip the bars too tight. Gearing: I've ridden it on both standard and compact chainsets. The biggest issue you'll encounter is the sea of people on climbs like the Paterberg and Koppenberg - practise (1) trackstands and (2) shouting "huphuphup" at people to get out of your way.
Last time I went, I did the full edition (265k). We stayed in Bruges (as that's where it starts).
When I did the 140k, I stayed in Brussels and just drove out to Ninove for the start (about 20 miles/30 mins drive). We stayed in the Hilton at the astonishingly cheap rate of £21 a night (incl 20 euros worth of breakfast..!).
I've only ever taken the car ferry from Hull, so don't know about Eurostar. A friend who rode the biggie with us last time took it and all seemed fine, though I didn't ask any detail. You may well have to bag the bike up.
Make sure you stay for the race the next day. It really is the most fantastic day you can imagine (once described to me as being like Wimbledon, FA Cup final and Ashes all rolled into one - it's a religion in Belgium). We usually go to the Muur, but as that's all change for next year I'd suggest one of the other bergs. Oh and get there early.
I've ridden a few UK and European sportives, and this ranks far and away number one for me. If I didn't have a small child and hadn't been off the bike for much of this year, I'd be there again. Enjoy..!