I found a 1991 escort lx for sale for $1200. I do not know how many miles it has. How many miles to much? Is this a good buy?

Standard or automatic?
From what it seems, anything under 100 thousand miles is good.
If it's an auto, and has over 100 thousand, check the transmission fluid, and smell it to make sure it's not burned at all.
I've gone through auto trannys so far, so I really would make sure that works before I buy it.
Try to test drive it, and listen to it when it starts up for any knocks or pings, check the oil; see how dark it is..
I'm sure more people will have better things to tell you