I think a spin bike, but one without pin detents for adjustments, but adjustments like the Serotta would be good. A Velotron eddy brake style rear wheel would also be nice. It acts as the flywheel, resistnace unit, and PM all in one.
I'm not so weary of the hi-tech fit bikes. To me they are an economical way to make rapid adjustments on the fly. They aren't that expensive to manufacture (any decent welder/fitter would be able to fabricate one for you for the cost of a normal decent road bike. For a shop this can be amortized over many customers, so in the end, in the hands of a good bike fitter, the small investment in time and $$ can really pay off. A low tech bike can bring the same benefit in the end. It will just happen a lot slower, and may be arrived at over multiple sessions. At the same time, I believe a bike fitting process is an evolution. It doesn't happen over the course of one fitting. I had a coach who tweaked my position 5 times in the course of a year, and two the next. It takes time to adjust to a position, even miniscule adjustments. Often times, a new position will result in a detrimental performance value in the short term, only to rebound to better than before. If done multiple times with positive results, you can make great leaps if you're willing to sacrifice a season.