OP- I've had my "old faithful" for ten years now, and I'm pretty sure it's at least five years older than that. I did what you're planning to do with your ride. It was a "big box" rigid mountain bike with pretty low end parts. Over time I've fitted it with slicks, new wheels, fenders, drop bars, clip pedals (I like the Power Grips for the variety of shoes I can use) a rack, and enough lights to make my Christmas tree envious. It's never been the fastest ride on the road, but it's fairly quick and my most comfortable ride. I can keep at least 18 mph in good conditions on it.

The advantage to this ride is that I can always switch out to knobbies for off road, and I have plenty of clearance for studded tires in the winter. I converted to friction shifting early on in the process- giving me the advantage of no cheapo plastic shifter parts, but more importantly as I wear out my cassettes I don't have to worry about replacing it with the same or similar parts. If I want 9 gears- fine, 7- fine. The bike is as flexible as I want. I put 3-4000 miles a year on this ride. The only thing it really needs now is a new paint job.

Bottom line, name isn't everything in a frame. Look for good construction and geometry that fits your needs and it'll serve you well. I'd bet that fellow with the used bikes probably has something for you.