The other one I was looking at was an 18 yo mare, I think that getting
an older mare to come into heat can be difficult, especially if she's
not been bred. I imagine not having a stallion around would not help
to induce her either. That's my fear, having an old nag around that is
not going to come into heat.
Now the 8 yo would be good to work with for some time and could
produce some offspring.