Try a PPS-43 next. The kits are about $70 state-side, plus another $100 for a receiver flat and trunnion and it should make for a cheap and easy build. If you want to manufacture and sell SMGs I think the '43 is a great model to build. Assuming you have an endless supply of cheap kits and the necessary jigs, then it would probably take only a couple hours per gun. Sell them for ~$500 and they would sell like hotcakes.
The ppsh 43 kits are cheap in the US for good reason, the reason is that they are expensive to make into a legal gun. None of the kits I have seen come with barrels, and by the time you get them up here, IF YOU RECIEVE THEM AT ALL, they will have cost a lot more than that. That unfortuinately is only the start of the problems.
The next thing that comes to mind is how the hell do you make one of these things semi auto without using a closed bolt striker system ? The Americans use remington shotgun trigger groups for these which is another expense. I don't think it would be possible to build one for $500 let alone sell a complete and registered one for that.
The next problem to overcome is the issue of what part is the reciever / frame ?

If it turns out to be the lower ( and I think it's quite likely it would be ) it would not be worth using a kit at all, you would have to make the whole gun.
Apart from all the above I would not consider making closed bolt guns out of open bolt ones as a matter of principle.
The suomi's are complex to build because they were never designed to be a cheap mass produced weapon. Every one is different, for example it is not possible to take a barrel from one and just stick it on another, everything has to be hand fitted to the reciever. But the nice things about the suomi's is that they are easily made semi auto only, without using a #### in the tube affair, and there is no doubt in my mind which part is the reciever.
PS, I think stens are the way to go to make a nice more affordable smgs. They are rough and ready, but they have great character and nostalgia, everyone knows what a sten is. Tell someone you have a sten and they would most likely say "cool," tell them you have a ppsh 43 or a suomi for that matter and they would probably say " a what ? " I have some good ideas for open bolt stens, like a mirror image one for starters.