I'm with Vorsprung regarding Merino. The thin versions work very nicely even in the hottest of summer (35 degrees plus).
As for Milecruncher's baselayer question: I think general advise is really difficult to give, as it hinges on your personal ability to cope with low temperatures as well as the actual weather during the event, and of course the rest of the clothing you are wearing. It may be too much, it may be ok, it may be too little. As others have said, layers will be your friend.
I'd be skeptical about any advice Brits give you about temperatures. Everything above 25 degrees is quickly considered a "heat wave" in this country, and even in December you can see posties delivering mail in shorts (seriously!). For whatever reasons, British people tend to have a much higher tolerance for cold temperatures than continental Europeans.
I am a continental European and think I have a moderate tolerance for low temperatures. As a reference point, here's what I wore on a 400 Audax in East Anglia over the last weekend:
- base layer: Icebreaker Meriono tank top (150 g fabric weight)
- short sleeved lycra cycling jersey
- short bib shorts
That worked fine during the day. At around 8pm, I put on merino armwarmers, and a windstopper gilet. After sunset, I added my PBP reflective vest and legwarmers. After midnight, I also added a long-sleeved merino jersey (probably similar to your merino baselayer), my waterproof rain jacket, long windstopper gloves, waterproof Gore socks on top of my merino socks (against the cold) and a merino buff. Temperatures at night dropped to around 9 degrees, and the kit was just enough. Had it been less, I would have wished for another layer, especially if you think of having to fix a mechanical in the dark. Or if it had rained. I'll most probably carry all this kit plus one more layer (a short sleeved merino jersey) on my bike on LEL.
Cycling in crocs? Just don't even think about it. It's unlikely that it will rain 12 or 24 hours non-stop. Your feet might get soaked, but if you carry spare socks, and maybe waterproof socks (which won't keep the rain out if it chucks down heavily, but will still keep your feet warm), you should be fine. Top tip for dealing with wet shoes: stuff them with newspapers while at a control - they will dry out much quicker.