I've virtually "built" several versions on IRO's website, and to be honest, I'm this close to placing an order even though I'd have to wait a couple months. It's just preferring disc brakes that's holding me back... although I don't even need to ride a bike at all when it gets crappy outside...

Like Ins55 says, a cross bike with a flip/flip rear and fenders.
If you don't like it you can always slap on a rear with gears, add derailers and your back to a geared commuter.
Oh yeah, don't forget light for the rain and fog.

Of course, you could do the same with a mountainbike. I did, but mine is fully rigid, no boinging for me. But I also don't ride trails.

Here is the wet weather commuter I am putting together:

Looks like the human-powered version of the BMW C 1.

The only Winwood disc forks I know of are muy expensivo. I was looking at something like the Surly 1x1 or Instigator for the same type conversion.

How about the Redline d440 29er?

Pros:
-It's a mountain bike, so you can do all of your off-road stuff
-It's simple: 1x9 drivetrain and no suspension should give you more gearing than a singlespeed but still be very low maintenance.
-It's a 29er, so you can put narrower 700c tires ont he stock rims and swap the knobbies back on for trail riding.
-It's cheap for what you get (decent steel frame, mid level parts). I beleive around $550.

I'd avoid other mountain bikes at that price range, having sold many. Pretty low end stuff, bedspring forks, etc.

Thanks. I will be happy if it keeps me warm and dry (and safe). I am mounting turn signals and lights now.