The Cornell multiplex absolutely shows whether there is an acute infection vs a chronic one, so it is a useful test.
If the horse is in the Northeast I’d absolutely do a Lyme test on a PPE but it wouldn’t affect my buying decision one way or another. I’d only be doing it to have baseline data. If the horse was acutely positive I’d just treat it if I liked everything else about it. If the horse was positive AND feeling great when you were trying it, I’d buy the horse on the spot since that horse would be a saint. LOL