I found an online "Roll of Years" somewhere once, and naturally I have since lost the URL.
There's a timeline of Faerun on
Wiki (of course). It looks like that Tuigan invasion happened about eight years before BG1, and got as far as at least Rashemen before it was stopped by an alliance largely from Waterdeep and Tethyr.
The Tuigans were not Kara-Turans of course - I get the feeling of more of a Mongol (
a la Genghis Khan) horde, but they had invaded Kara-Tur the previous year. In any case they'd have exotic weapons of some sort, and probably quite a few Kara-Turan weapons. And the invasion may not have gone as far as the BG region, but it's certainly plausible there would've been enough trade in 'souvenirs' as you say, to satisfy any screams of 'NOO! Katanas and wakizashis are ultra ultra rare on the Sword Coast!" Bleh.

Having said all that, I'm leaning toward our mystery monk being from Estagund. It's close enough to the Sword Coast where he could've conceivably joined a caravan to BG or Athkatla as a guard, and decided to seek enlightenment at Candlekeep afterwards. And the
description fits in pretty well:
Estagund: "Durpar, but with warriors exalted over merchants. Noble slayers, but slayers nonetheless -- and dangerous because of their honor and discipline. This is where the Durpari get their guards. Wealthy but with more places to ride hard, more walls for defending, and fewer gardens. They love beautiful weapons, and buy and sell such things as eagerly as children devour candy..." (It is modeled after an Indian culture mindset.)
So I'm thinking this guy has some decent weapons from his homeland, nothing fancy, pretty commonplace to him, but decides he really wants to swap out for these fancy, exotic (to him) long swords and whatnot. Maybe he's an outcast
Marquar crusader (dunno if I want to run with that though). He's also a bit of a scholar and has a book or two from his homeland (which could've explained any entry requirements - again, what may be common to him might be considered rare to Sword Coasters). And my work is already cut out because there is a 'History of Estagund' already in the Candlekeep Library. Where did they get it? Ok, maybe it's not all that rare, but still.

It also seems a bit absurd the only 'exotic' weapons are Kara-Turan. Have you *seen* a
map of Faerun? Place is huge, and there are a lot closer 'exotic' places to the Sword Coast than Kara-Tur.
Rather than just stick his weapons on Winthrop, I might even introduce the Estagundan himself as a CRE, give him some dialog and make him a store so's CHARNAME can buy his weapons (if the PC isn't too insulting), if I'm feeling crazy.

So I'm thinking he might have a
dha, a
shamshir and a
kastane, in lieu of a katana, a wakizashi and a ninja-to. They'd be either -1 dmg (so 0-9 for the
dha) or slower than their counterparts, or both.
I promise I won't give him the voice of
Apu from the Simpsons.
I don't want to get too carried away but what if, 'and believe me this is hypothetical' (
says in his best Bob Slydell voice) I was to introduce shuriken (throwing stars) as rarities? Which proficiency should they be (without inventing a new one of course) - throwing axe, dagger, maybe dart? Any thoughts on damage or placement?