Converting an existing cyclocross bike to disc brakes I have a bike (Surly Crosscheck) that is used for commuting and cyclocross. As it's a bit older and disc brakes were not legal for 'cross until recently, it has cantilever brakes for optimal mud clearance.

They also kind of suck for commuting in a rainy city with panniers.

I have been debating doing a conversion to discs for this bike, but I don't really know what I'd need to replace, so a list would be highly helpful, then I can price it out and decide whether or not I should just buy a new bike... Possibly I could also keep the cantilever on the rear and just go with a disc on the front for good stopping.

Presumably the wheels and front fork at the least would need to change. As would the brifters as those probably pull differently. What else would I need?
Hydraulics for your drop bar bike might be difficult to acquire. I believe that someone made a cable actuated hydraulic brake ... it was downright terrible. 

The clamp on adaptors are in my experience a nightmare. They allow for greater vibration and therefore terrible noises. 

First, check your bike for disc tabs. If it doesn't have them you could have them welded onto your frame by a local builder. As well this could be your opportunity to upgrade your bike. Haha. As well it might be cheaper to simply purchase a new fork rather than have tabs welded on. Who knows?

Second, check your wheels. Do you have disc hubs? If not you will minimum have to rebuild your wheels with a new hub. Ensure that you can build the wheels in a nice dependable 3 cross pattern to resist wind up. Again, this might be the time to get a nice new set of wheels. Or the opportunity you have been looking for to start building your own wheels. 

Thirdly, what shifters do you have? Does your crosscheck have barcons w/ independent brake levers or STI style shifters? If you have STI style shifters then AVID makes a bb7 road disc caliper. This caliper has had the pull adjusted to accommodate road set ups. If your setup features independent brake levers for the calipers the road bb7 is your best choice as well. 

That's about all I've got for you. If I can think of more ... I'll be back.