Call the sellers and see what neg holders and lenses, if any, are included. You'll probably find one that has a 6x7 holder and suitable lens. Glass holders with cropping blades are ultimately flexible and perfectly flat but really hard to keep clean. Glassless holders are trivial to use but can suffer film-flatness issues in some enlargers, particularly if there is notable heating of the negative.
You need at least an 80mm for 6x7 unless it's a special wide angle enlarging lens (eg Rodagon-WA; $$$$). 75mm is really only for 6x6 and some will insist that no less than 90mm is necessary but that's IMHO quite conservative. Shorter lenses (assuming enough coverage) give bigger enlargements for a given enlarger height, so don't buy one unnecessarily long like 105mm unless you like making smaller prints: the longer lens will give you more room for dodging and burning. Get a 6-element lens (Rodagon, Componon(-S), some EL-Nikkors; probably $50-$100) not 4-element (Rogonar, Componar; $0-$20). APO is nice but not worth the $$$ at this point unless you find one for non-APO pricing. I'd be really surprised if you couldn't find an enlarger with lens included for the value of the lens or less.
(I use a Componon-S and Rodagon 80/5.6, they're sharp enough to render the grain clearly on fine films at 16x20")
If you have a choice, get an enlarger with a colour head (not LED, not cold-head, a basic incandescent with dichroic filters), it makes life easier for B&W too (no separate contrast filters that might be faded)... and you can print colour later! A 4x5" enlarger is likely to be sturdier but obviously will take up more room. Consider also bench top vs free-standing, and the ability to project horizontally. And ruggedness: go look at them and have a feel, look for abuse; they must be totally straight.
My FAQ (see my signature) may be of some use and many of your questions are covered nicely in WBM2.
Last edited by polyglot; 06-17-2013 at 10:33 AM. Click to view previous post history.