What's My N+1?

A CX bike would probably work well since most have fender and rack mounts.  You could also use a CX bike as a road bike simply by swapping to road tires.  Your position will be somewhat relaxed due to the nature of CX bike geometry, but it seems low on your priority list.

My personal opinion (as some one who has 8 in his stable right now, and is currently building the frame for #9) is that if you plan to do much of either xc or crits/road races, you're really looking at N+2, not N+1. Yes, you can do road races and crits on a 'cross bike, but it's a "can" and certainly not ideal. Yes, I can point out several people who do just fine in the roadie scene on a cross bike, but there's a reason that the two bikes have differing geometries. But I'd also not enjoy an 'cross race on a road bike, even with 28mm tires (regs limit tire width for true 'cross to 33mm, I think, but there are few road-race oriented bikes that allow that kind of width -- Volagi being one of the standout exceptions).

If you're really stuck with it being +1, then I'd go with a 'cross bike that allows for the addition of fenders, throw some beefy road tires on it for the commute, and rock it all winter. The Kona Jake is a great choice for something like this, though there are several other similar models in the same price range. Then I suspect you'll be looking for a road racer in a year or so...