Usually the UN will sell or give older designs. Sometimes it may be better than what you have but usually its not. If you can afford it buy it make sure and then double check. After a year or two of playing the game you will pretty much know what your getting. Until then its all part of the learning process.
Far as what units are better, it all depends on what you want them to be used for. For example:
Infantry for defense , you will want a unit with a high close defense and ground defense values. Now add in if you prefer it too be legged or mounted , amphibious, etc,,, then choose the best unit for your playstyle and strategy.
For Arty you should probally go for the longer range variations as much as possible.
For offensive type units , you will want units with a high Hard attack and/or Close attack. Hard attack is for hard targets like armored vehicles and close attack is used for attack in places like cities or close combat hexes.
For the most part , you just need to play around with the units. You can do very well even with obsolete units if your patient enough to plan ahead and use your units to best of thier abilities.