They are great dogs... Just be aware that as a breed they do tend to be a little inclined to "nip" as this is a characteristic they were bred for, as a working dog.
Because they are working dogs, they need a big yard, lots of running space and exercise...
Another few tips (that goes with buying any dog) check out the breeders and make sure you see the parents of any pup you intend to buy...
Anyway, hope this helps a little...but ACD's are awesome dogs...
Out of interest, are you looking at a blue or red? Stumpy or tailed?
Not that it makes much difference

just me being inquisitive!
Love & prayers as you do your research and hugs to your aging companion!
Lisa