To print 35 and 6X6 you need a lens that will cover the negative. 50mm is considered normal for 35 and 75 or 80 is considered normal for 6x6 and 90 for 6x7 or 6x9. Some printer (people not the enlarger) use a longer lens than is normal such as 60 for 50 and a 105 for 6X7 or 6X9. There are many very good lens on the market, I use Wollensak, Nikkor, and Schnider. Buy the enlarger then check to see what lens and lens mounts you will need. Buy the best lens you can afford. I would use an older enlarger in order to get good lens. If the trade off is good glass vs the basic enlarger I would look at a Federal or Dever, any enlarger is only as good as the lens.

Paul