If it's worked for your centuries, I don't see why it wouldn't work for a 300km. If your bike doesn't accommodate a rear rack, you could get a larger seat bag or handlebar bag to hold some stuff, like Maillotpois suggested. Compact double vs. triple depends on the terrain and your legs. It's a big expense to switch from a compact double to a triple, though.

I'd say, use your current bike and if you really get into randonneuring, you'll know what to look for in another bike later.

BTW, I have not done a 300km ride, and since I'm not fond of riding in the dark, even with lights, it's not likely that I will do one. But I sure admire you and others who do them!