You asked about "steampunk" monsters in another topic.
To make steampunk characters is something else. And epic? Wow, you are ambitious.
I guess it really depends on the vision you have for the campaign. For example the Girl Genius comics paint a picture of a world dominated by the "Sparks", the mad scientists of the world.
Other visions, such as those of Jules Verne, had the occasional "madboy" type, but they were always singular, the hero or antihero of the tale. In a game they would be NPCs, more plot features than actual player characters.
The Girl Genius type campaigns would have every player wanting to play the mad scientist. That's a hard campaign to run or manage, since the nature of the madboy is that he/she dominates their own personal world. They aren't "adventurers" in any normal sense.
All of that being said, if I were to try and do a Steampunk style game, and I was bound to D&D/D20 mechanics, I'd take a serious look at one oft overlooked rule in the books: Spells can take on just about any appearance, and can be different for different casters or disciplines.
So, does your "fireball" resemble a steam explosion? Is your "lightning bolt" called a "Death Ray"? Okay, that's cool, we can work with that. If your Staff resembles a ray gun with a backpack, what of it? It's all just game color.
The one big obstacle to mixing the industrial age with the time of legends depicted in magical fantasy is that magic is only available to the few. Machines, once designed and built, are available to anyone, whether they understand them or not. Wizards and Clerics run out of spells, wands and staffs run out of charges, but a rifle can keep on firing for hours, dealing for more death at a much greater range than most any spell. So your Wiz can summon a Mount spell? My motorcycle will run faster and farther by a factor of two or three, easily.
So a certain amount of window dressing can be applied. Your "Wizard" prepares his "spells" by stockpiling strange reagents and rare chemicals into a bandolier, and your Flying Carpet looks a lot like a dirigible. But other aspects of the setting get harder and harder to develop as window dressing for standard classes, spells, items, skills and feats.
It's like Vise Grips: They're the wrong tool for every job! (you can use them for just about everything, but they're never the "right" tool"). So it is with D&D/D20. You can use it, but there are many better choices.
Consider the Savage Worlds system. I'm told they have a decent Steampunk sourcebook.
