One point I think that should be made is that the kind of assassins Xizor can hire to go after Revan wouldn't exactly be the kind he hasn't faced before. They wouldn't exactly be something that would catch Revan by surprise.
This isn't strictly true.
The only "assassins" Darth Revan has faced before are Jedi. He will be completely unfamiliar with other non-force users and their tactics. IG-86 droids did not exist during Revan's time. I'm almost positive he's never had to deal with an Anzat. Let's also not forget that Redeemed Revan fell prey to many assassination attempts by non-force users, and was almost defeated in several, had it not been for his companions. However, these attacks haven't happened yet, and so he will still be prone to underestimating them.
Darth Revan has not faced assassins other than Jedi, so he will be just as "surprised" as Xizor.