Very cool idea, and I wish you the best of luck!
Some thoughts of my own (take them as being worth what you paid, however...).
I agree with others that there are other variations of the Martini that would be in more demand for new-built actions... however, I respect your wanting to wet your feet with the more routine action first. I might choose the same path, if I had the wherewithal to get into such a project.
12L14 would make the manufacture way easier (it machines SOOOOO nicely!). However, I too would be a bit concerned about it's strength. If it were me, I'd consider 41L40 instead. OR, have a pack-carburizing* and hardending/drawing be the last step in manufacture. But by that time, you may have passed the point of economic value.
* For those not familliar with pack-carburizing, it's like standard case-hardening, only the case is FAR FAR deeper, up to 1/16" of high carbon surface.. the process takes about 12 hours in an atmosphere controlled furnace packed in carbon bearing material.
I would want to sit down with someone who is passingly familliar with action-engineering from a metallurgy standpoint, and make sure 12L14 is suitable. I don't know that it's NOT... just would want to be safe.
Also, if you were to eventually build "german style" Schützen Martini actions with double-set triggers... I would SO be buying one from you!
Keep us posted! I'd love to see ANYONE making ANY "new" single shot action for the market!
Paul F.