you need to consider which formats you are now shooting and which you may want to do in the future. The Beseler 23C mentioned above is a very nice enlarger; however if you ever want to print 4x5 negatives that enlarger will not support that format.
A Beseler 45 will.
There are several well built enlargers available for very little money these days, and the age of the enlarger if cared for doesn't make much difference.
There are Omega D's ; and old work horse , found in many pro printing shops. Omega-Satter LPL are very stable and a nice piece of equipment.
There are a variety of lightsources that can be switch out with the older enlargers i.e. cold light heads , color heads and condesnor heads.
A color head can be used for black and white, and since you have indicated you may do color in the future that is something to seriously think about.
some of the newer enlargers have VC dial in heads and it becomes very expensive to swith those around.
Nikkor makes very good lenses, especially with the 6 element glass. Schneider and Rodenstock are very well know makers of lenses.
With Schneider, look for componon lenses as they are the top of their line.
Any 2.8 lenses is better than the F4 with 50mm lenses.
a 120 lenses is a strange size, 135 or 150 is used for 4x5 , 80 or 90 will give better coverage for 6x6 negatives, altho a 75mm can be used.
100 or 105 will cover 6x7 or 6x9 negatives.
CHeck the newspapers in your town to see what may be available. THe shipping of enlargers are expensive, and it would be good to see the enlarger up close and personal. Check to be sure the bellows is light tight and that the negative stage is in alignment. You can over look at bit of dust and rust as older equipment can show it's age, but if the bellows or condensor are bad, you looking at replacement cost which can put a dent in the wallet.
Find out what negative carriers come with enlarger, and if they are offerning to throw in a lens, is it mounted in a lens board , and/or does the lens include a jam nut.