I have 2 Singers, an Elna, Hobbylock and Juki serger. All but the Juki are 4 thread. I recommend the 4 thread because you get that extra solid line of stitching. They all work great. The Elna requires more oiling than the others and that can be kind of a pain.
I had a new White (the gal only used it twice) but didn't keep it. They still use the old threading technology, which I found very annoying.
With the exception of one of the Singers, I purchased all my sergers used. I have so many because I teach sewing and own the machines in my class. My kids have had no problems learning and using these machines.