Top of the line are Barudan, Tajima, ZSK and I don't know if they are still made but Happy and Toyota. After that SWF, after that I wouldn't touch them. Mostly made in Japan or Germany is top line. South Korea next. Stay away from the Chinese made machines.
If you want a single head, get a full size, not a compact. But I wouldn't recommend a single head. For the price of a 2 and single head you can get a 4 head and turn out much more work. Now, having said that, having a 4 head and a single head can make sense. It does for us because we do a lot of letterman jackets and it keeps the 4 head free for larger jobs.