"It depends what you're doing with the machine..."

I think, like most people, I want something that is accurate and repeatable to within 0.000001" and can easily and cleanly mill everything from paper to adamanitum, while costing <100 to build

PCB milling and aluminium/acrylic sheets for panel making is high on the list. The X/Y work envelope of 10"-12" is based on this. Small aluminium pieces (by small I mean 1"x1"x1") would be very useful.

I would really like to work on larger aluminium pieces, primarily in order to make things to improve the mill/build the next, but also for other occasional projects. For this reason I'd like 5"-7" inch Z axis, though I appreciate this is quite significant for a small DIY mill. I might end up sacrificing this functionality but it would be nice to have it.

I'm working on the basis that if I can build a solid enough design then I can work on a spindle motor solution (swappable mounts? pulley system? variable speed motor?) that will let me cover the range of work. I'm also not too concerned with the overall process speed - this is for personal projects. Fine if someone else can do the job in an hour start to finish. If I'm spending 30 minutes for each cut and the whole job takes a week of evenings, such is life.

Yes, I would like to buy once/cry once in terms of ballscrews/nuts etc, but I've weighed it up carefully and it's not within my reach at the moment.