There are many breeds who can do agility, not just border collies. Lol, i've seen some Jack Russells tear up an agility track!! Many types of dogs love agility.
But, you are right------many border collies do rock at agility.
Since you asked, and since you DO have a specific hope for your next pet,
i myself would not get a puppy.
Lol, i'm too lazy to raise a puppy for one thing, nor do i care to engage in the rug shampooing and replacement of various items that can get destroyed during puppyhood. I'm not up for the up-all-night crying stage, the nipping stage (OUCH!) the ruin your carpet stage, the "can't be home alone too long" stage, etc etc. and all the other stages of puppyhood that don't reeeally fit my life if i can avoid it.
To me, puppies are a mystery-box of 10 to 20 years of a dog you *maybe* did not expect!!! ha ha!! YOur dog could grow up to be a couch potatoe, or a speed ball, and either one of those might be a disappointment. My guy's last puppy, turned out to be 3 times the size he'd expected, lol, and half as energetic as he wanted. But, it sure was a cute puppy!
RE: puppies,
You CAN spot a super-shy puppy in the litter box,
but a dog-aggressive dog does not display that neurobiological disorder til about 9 mos on average. A few report later onsets. but, typically, by one year old, the condition is
obvious. Making a one year old dog a nice age to look at.
I'd stick with Petfinders, i do think you are on right track, adoption is the best way to go, imo, for both moral reasons, and for dog overpopulation reasons.
I myself would go for
at least a one year old dog, but, that is just
me.
That way, you can see WHO the dog is. With puppies, there's uncertainties there.....
.
.
On petfinders, you can look at
both
dogs in shelters
,
and
dogs in rescue.
.
With dogs in shelters, you can have the dog temperment tested, but, really, no way to know if he'd be good at agility, and
i do believe,
some of a dog's ability to "be good at" agility,
is in how they are introduced to the sport,
and how they are trained at the sport.
.
Dogs in rescue, are usually living with a foster family.
That foster family can and will tell you everything you want to know about your dog,
is he energetic?
Does he learn cues easily? (take
that one with a grain of salt *if* the fosters are not adept at teaching cues, lol).
Does he get on well with other dogs? cats? kids?
does he get on well with humans, or, is he a fearful/ shy type of dog?
What games does the dog enjoy?
How much excercise does THIS particular dog need to be sane?
How long can he excercise before he tires?
Does the dog show any aptitude for agility? Has he been trained or even exposed to the sport at all?
.
Just whatever you wanna know,
the fosters can tell you,
so you can bring home a well-suited, no-mystery dog that will fit well into your life.
Adopting a dog in rescue takes a suprising amount of
months, there are
many steps in the process.
Telling the rescue org, you are hoping for a dog who will enjoy doing agility, can help them locate which of their dogs who might enjoy that.
If you have never done agility, it might be great idea, to google your town's name, + word "agility"
and go to a meet,
and talk to those who do agility wiht their dogs, to get an idea of how much training is involved, etc etc.
Even though i do believe, many dogs can enjoy being trained at agility,
and some of that is
HOW the dog is trained,
still, you probably should also assess
,
if if if it turns out, your particular pet is NOT great at agility,
will you still be committed to loving it unconditionally and caring for it for life?
Is the dog
winning important to you, or just
participating for the fun of it is what interests you?
It's a long term commitment.
I
admire your willingness to provide a dog
a great life,
but always good to consider--------- on the footsteps of a long term commitment, ------------to truly assess what all your expectations are, and how you'd feel if those are not met, is good to be self-aware at this stage of the decision.
BEST OF LUCK!! Getting a new dog is exciting,
keep us posted!! 
I was looking for a younger dog, not necessarily a puppy. I don't care about a dog being a mix, just as long as they have the qualities I want. 
i did not realize, this will be your third dog!!
A dog-aggressive dog, sometimes "lies low" for that initial "new in the home" stage.
I can tell by the other dog's personality if they would get along. Shiloh is the one I would have to worry about. She is the big sister and wants to be in charge. Another female dog like her would not mix well..... Other than that, I have no concerns about her and the other dog. (This dog will have to be good with other dogs. I worked with Delilah and my foster beagle with fear of people, but I can't work with any DA or PA dogs.)
Not very desirable eather
