Thanks for all the info. I will go with the 5 speed... If I don't find it in the US I'll just wait the long shipping from UK.

I found a SA 3 speed (NO coaster brake) in my dad's garage and I was going to try it with this bike but the frame diameter is bigger than a normal bike the cable guides won't fit on the frame. I had the same problem with a few accessories :/

Is there any way I can use any other DIY guide for this frame?

I won't have the same problem with the SA 5 speed hub