The DK mount is completely worth buying, if for no other reason than it looks better than most of the other flying mounts you can easily obtain. Once you get your hands on a Frost Wyrm or something I imagine you won't be using it again, but in the meantime it is part of your character's identity and looks much coooler than a Windrider. The only immediately obtainable mount Id consider otherwise would be the Cenarion Hippogryph, but the effort involved in that is much, much higher and it costs a fortune.
As for skill... trust me, you NEED Epic flying skill for Icecrown or Storm Peaks, there just isnt another option really. In TBC you could get away with the normal speed mount because most of the travelling was done in a flat plane, and you only needed flight to get to certain places like TK; this was essentially an artificial progress barrier, and Epic skill was more for convenience.
In WotLK however, the high-level zones seem designed with Epic flight in mind; without it, trips to quest areas are agonizingly slow air-swims that can take 4-5 minutes, rather than a speedy little 30sec jaunt on your nice quick mount. Remember also that On a Pale Horse applies to 280% mounts, so you will be going at a truly phenomenal speed.
Just save the gold and buy it; there really isnt a good reason not to.