fijfi,
First of all, the hammer. Look at this pic.
[img]http://us.f1.yahoofs.com/users/c661b06/bc/Sten+project/ar15+hammers.jpg?bczrW_9AVRuRrJiL[/img]
The hammer on the right will work w/ the 9mm. If your hammer looks like the one on the left, with the hook, it won't work. Don't even try it, as it will lock it up....so I've heard.
Okay, now..... I have a Cav-15 and I'm using the stock 223 buffer and spring. Works fine for me.
Mags, the factory mag well adapter is for Colt type mags. Certain ones have "issues". Hard to say which ones. A trick/tip. If you find a mag that is loose, getting some aluminum furnace tape and putting it on the non mag side can tighten them up. I did that to a few mags and got them to work fine (range use).... this was before I did the Sten project. You can find the aluminum metal tape in the furnace section of a large hardware store. This is NOT duct tape.