Purchasing a used 20" ACD 3 years old question

I am thinking of buying a used 20" ACD that is 3 years old.

Therefore no Applecare and 2 cosmetic blemishes that I can live with. No dead pixels.

My questions are:

1. The fact that it is 3 years old, has this ACD seen critical revisions? (LCD Panel Upgrades, etc...)

2. What do you think a fair price is for this ACD as described?

Thanks in advance...