Collies are awesome PSDs. My first PSD was a collie (Wings). When she passed away, I began training Mal, a smooth, who has since washed out of training. His breeder donated his half-sister to me, though, and she's showing good potential so far. (Mal is barky- the barkiest dog she's ever bred, apparently- and this was a factor in washing him out. Before I got him, I had never dealt with a collie that was so bad about attention barking, but they *are* out there.)
Going with a good breeder is important. The CCA and AWCA have a pretty good emphasis and recognition program for service dogs, and many reputable breeders have produced dogs who go on to become service dogs.
The smooths and the roughs can vary in personality but in my experience- and I've got less experience with smooths- they're not THAT different. (They may vary more from lines that are all-smooth or all-rough.) Mal and Kaylee are from a breeder who breeds mostly roughs but not exclusively and Mal's sire is a smooth from a breeder who is known equally for her roughs and smooths. I am told the smooths are more energetic overall, which doesn't seem, to me, to be true, but again, my experience is limited. Mal is less energetic than Wings was and I suspect Kaylee will be in between the two. The smooths shed a moderate amount, all the time- the rough-factored smooths seem to shed more than the ones that are from all-smooth pedigrees. Mal gets REALLY hot out in the sun- at least as hot as his rough sibling- and I think it's due to color and the fact that he's got just as much undercoat as she does- it's just shorter and flatter.
Kaylee has more hair than Wings did (Wings was from working/pet lines) and the texture is different. She takes more work to keep looking her best, but MUCh less work to keep her from dropping hair all over the place. So there is a trade off- grooming time ends up being pretty similar on both.
I get much less random pounce-and-hug type problems with Mal and Kaylee (who are both tris) than I did with Wings (who was a sable)- and pounce-and-hug-kids are a HUGE distraction for all the collies I've met, so having a dog that doesn't invite that QUITE so much is nice.
Mal is 25.75", right at the top of the breed standard. I would not want a dog much larger than him for a PSD, as he's a pain to fit into places. (He also doesn't curl up very well, compared to Lindsay's Caden, for exmaple, who is 24ish"- those two inches make a BIG difference in how much space he takes up.) Kaylee is 24" and that really is a nice size, IMO. Wings was a bit smaller- 21-22"- and fit even more nicely into tight spaces.
The lifespan is good, and the main testable health problems are PRA (we JUST got a DNA test for this) and CEA. CEA is the bigger deal- MOST dogs are carriers, but even affected dogs will vary a LOT in degree. We've also got some other problems that CAN'T be tested for- so ask about them. These are bloat (scarily comomn in some lines) and epilepsy. Epilepsy is what killed Wings. HD and ED are present but not terribly common, and some lines are known for thyroid problems and allergies.
In short, like most of the breeds that have been popular at one point, collies have a nice variety of health problems, but also a really great core of dedicated breeders who ARE doing their best to breed happy healthy dogs. The collie temperament is really well suited to PSDs in my opinion- they're handler sensitive but not necessarily to the degree of being neurotic (some can be- this is to be avoided, obviously), they generalize well which makes them fairly easy to train, and their energy level is moderate, which means they're suitable for someone who's ability to exercise them may change from day to day. They're less 'busy' than Aussies and BCs, for sure. Lack of drive *can* be a problem, but not all of them are this way.
If I were adding another collie (Wait, I just did

), I would look for a young adult retiring from the show ring that had been well-socialized and health tested, rather than another puppy. Collie boys, especially, are slow to mature, and you can save a LOT of time. Bonding is NOT an issue- collies bond really deeply with their people, and while the bond doesn't transfer per se, they are PERFECTLY capable of bonding to a new owner who makes the effort to work with them.