I think it really depends on the resources you have available. I wouldn't be asking myself how far I could drive, but rather, how you are going to get the horse home. I'd drive a pretty good ways for the right horse. However, I already know I can't haul the horse myself. If you are having to pay someone to haul for you, then you have to take that cost into consideration. Most likely, the further you get away from you, the more expensive it is going to be.
You may be able to find someone that charges you a flat rate. Just to move the horses I already own from one barn to another (15 minutes away from each other) I got $5 per loaded mile and the cheapest flat rate I got was $50. This was for two horses. Also, sometimes sellers are willing to haul for you for a price. You should probably find out what you have available in regards to hauling before you go look at a horse that is very far away.