I've done it a couple of times and the pain really is something to behold. Worse than anything I've felt on a bike. (that's why I had to do it a second time)
I recommend double strapping your bars and 25mm should be fine for tyres. Make sure you are fit, because if you end up going slow over the pave then your life will be hell.
It is quite unlike other sportives, there is a lot of respect for the event amongst the faithful. No-one goes off fast at the start, they all know they have to preserve their energy.
If you are going to start in the dark, just take one of those 'pretend' lights. You know, the ones that have a single LED and emit the same lumens as a distant star on an overcast night. You need it to pass the [lax] inspection, but not really to see - and it will fall off anyway.
Make sure you take a towel so that you can wash your self in the same cubicles as every winner has for the last 100 years. The sense of accomplishment is unbelievable, but it is hard.
I'd love to do it again some day. Let us know how you go.