Get modern lenses - old lenses are not coated and that does make a difference. You can know what they look like and their names by visiting the manufacturer's web site. As far as serial numbers, there is no difference that I am aware of in the product run - too expensive for manufacturers to retool for a small change in production. Get a six or more element design - the ecomomy line uses a four-element design. APO lenses will let you work at larger apertures and higher magnifications, but I would be hard pressed to see a big difference in the results from them and the standard line.
I have Rodenstock Rodagon and Nikkor EL lenses and they are excellent. I have also used Schnieder. I doubt there is any real difference between the three manufacturers. Some of these lenses have been rebadged by Omega and Bessler and can be gotten for less on ebay because of the name - no difference in optics or design. I am sorry but I can't remember the names exactly, but I think a Bessler HD is a Rodenstock Rodagon.
A 50mm, 80mm, and 135mm is a good set for 35mm, 6x6/6x7, and 4x5 film. For 6x9, a 105mm lens is better. A 150mm is the standard for 4x5, the 135mm is for when your enlarger height limits magnification. There are fewer focal length choices for apochromatic lenses.