She should be PDd back in calf (just to make sure that she actually still is fertile) before you consider buying her.
Her feet and udder should be in excellent condition. She should have a safe and regular calving record.
I've bought 8 year old cows before with calf at foot and in calf again - one lasted until she was 12 and one is still going at 15.
But if you buy an older cow, you don't know how long she is going to last - poor feet, arthritis, poor fertility may do for her in the end.
Oh and make sure you see her last calf - if it's a poor looking thing, it may indicate she is no longer producing enough milk.
I don't buy in cows anymore but, when I did, I bought them in pairs.