Let's take the STEN first. If you build a functioning firearm using those parts kits, you need to be very careful as to how you make the receiver so it cannot be be made full auto. Usually that means receiver tubing smaller than the original so that the original bolt won't fit. Plus the mechanism has to be made to fire from a closed bolt, with a separate hammer/striker, not the firing pin in the bolt face. There is good information on the net, as well as books and a series in the Shotgun News on making up semi-auto carbines. (There is also some information and advice on the net that, if followed, can get you a free vacation in Club Fed, so be careful.)
Then, of course, if the gun has a shoulder stock, it has to have a barrel at least 16" long.
The M1903A3 drill rifles I have seen have tack welds that close the front of the bolt and prevent removal of the bolt, as well as plugs welded into the chamber, and the barrels welded to the receiver. Restoring those to firing condition requires breaking the welds, replacing the barrel and the bolt, and then headspacing the rifle, which could be impossible depending on the replacement barrel. There are no legal problems as there are with demilled auto weapons, but there are concerns about the effect of the welding on the receiver as well as the cost involved. IMHO, while the project could be a learning experience, I would not buy such a restored rifle.
Jim