Here is my build log: Granite surface plate lathe
I designed the lathe to be gang tooled with no tailstock. I will probably add some kind of tail stock for long thin parts but it would have been better to design it in from the start.
If you want things like waycovers, enclosures, chip pans etc they should be designed in from the start.
This is more into headstock design but think about a servo/stepper indexing the spindle maybe with a brake.
I built my project entirely on a fairly worn out bridgeport using pre-ground steel plate. Think about what tools people will need to complete the project.
A big question you need to answer is why buy (use) your machine bed rather than buying a 7x10 or 10x22. Bigger, smaller, lighter, heavier, etc.
What type of ways would you use?
One option I see is having a flat base with a precision shoulder to align a master linear rail. The end user could buy there own rails on ebay and drill mounting holes without needing precision tools.
I might also be interested in a scrape-your-own box or dovetail lathe kit.
What type of spindle would you use?
How about setting up the z axis and using it to bore the headstock. You do the design around and existing spindle and the user makes it.
What type of axis motor?
For small lathes I think the "crevice reamer package" G540 and keling steppers is a good fit.