"More horses rust out than wear out"
It would be unusual for a 15 year old that had an active career not to have some arthritis. As previous posters mentioned, the questions you need answered are how severe this arthritis is, and how it will impact want you want to do. Frankly, I've never heard of signifigant arthritis in an *elbow* - fetlocks, hocks and stifles are a lot more common. You will need a vet to examine her, do flexions and possibly xrays and give you an opinion. You will also need to add the cost of the supplement and shots to her cost to to give you and accurate comparison to other animals you're looking at.
FYI, arthritic horses do much better with turnout and a run in shed; standing in a stall for long periods does them no favors. They also do best with light, consistent work; they don't do well as "weekend warriors." I have also had very good results with joint supplements; thought you have to experiment a little - different ones work better on different horses.