stocks
Yes, you could adapt a folding UZI stock to the macs, but you could not shoot the gun with it folded because it would cover up the grip.
As for the Mp5 type stock...well then you run into your issue of cost. Those stocks are not cheap and then you have to find someone to fit up to your mac. I am not sure if it would work when collapsed either.
The cheapest best stock that I know of is the Autowerkes fixed FN type stock for about $110. Yes, thats not super cheap either, but good quality work and materials don't come cheap.
For folders you are kind of limited if price is a option. I would steer clear of side folders because most I have used don't lock up tight over time and won't let you shoot them when closed. Besides sidefolding stocks are bulky. I like the ares type UF but they are hard to find now and on the upper end of the price scale too, and to me a little short. I won't mention my UF because we have allready had that conversation before
Macs are just one of those SMGs that it seems you have to spend about the cost of the gun in assec. before you feel like you have something IMHO. Just FYI I have never known anyone who was dissappointed by buying the best, it is when you compromise that you have problems.
Best of luck, let me know if you have anymore questions I can help with.
Best
Chris