I've only seen Lajoie play in person twice, and that was years ago so I didn't notice him. I have a friend in Regina who watched him against the Pats and he's did well apparently. He had 9 points in 14 playoff games. If you look at his points, you'd think PMD, but that is not his game. He's more of a two-way guy. Think "poor man's Phillips".
As far as AHL goes, Lajoie will not have completed 4 full years of Major-Junior, so he'll have to be 20 before he can play in the AHL. It will basically be the same situation as Gabriel Gagne. If he REALLY wants to play in the AHL, he can play 20 games in the ECHL and then report to the B-Sens when he's 20 mid-November, or he can spend his last season in the WHL. Ottawa has a logjam on the left side with Harpur, Englund, Sieloff, and others so I don't expect them to rush LaJoie.
One interesting thing to keep in mind is that the Broncos have a real gem on their hands with Artyom (Artem) Minulin. He's a Russian playing CHL as an import like Sergachev and Provorov. He's expected to go in the late second or early third round and if he's around for Ottawa's second round pick, I'd take him. Hell, I'd trade up to take him. He's exactly what Ottawa's pool lacks and "should" be available. If Ottawa has been keeping an eye on Lajoie they will have had a good look as well, and they may like what they see.
As far as the draft goes, I'd also take a look at Jake Leschyshyn, son of Chris, he's expected to go in that same area in the 2nd round and plays for the Pats. He's stuck behind Sam Steele, but Steele has a good chance at graduating next season for Anaheim so Jake Leschyshyn will get the top line minutes for two more years. He's like Lazar but won't require a 1st roudner and you can let him sit in the minors for a while. The family connection is kinda cool too.
Only other guy from the WHL I see the Sens having interest in is Kole Lind. His point totals don't "wow" but he's a solid player. This season he was half-way stuck behind an over-age guy (Reid Gardiner, a Penguins prospect who played 4 years and spent 1/2 the season in the AHL) on the RW. Weird situation. He's 6'1, so his size is fine. Just a gritty two-way guy who can score. If he had better point totals there's no way he should fall to Ottawa in the 1st, but who knows.