Hmm, that sound's intriguing...
I've borrowed Piratecat's Eversink (city) from his Spira campaign world, which has a bit of Italian feel to it. This adventure sounds like it might fit well into that setting.
Hmm...
Well, it's been a long time since I ran it (as in, 3E didn't exist yet, and I Skills and Powers + Combat and Tactics were Brand Spanking New). I would never have even purchased it, as at the time I believed only "newb" DMs ran modules... the only modules I had were ones that my dad had purchased when he was stuck being the GM at his own games.
This (along with Isle of Dread and Tamoachan) were was one of the ones that broke me of that mindset. And it was one that we really enjoyed.
I'm digging through my copy right now, and a few notes pop up:
1) The city map is pretty much useless. Disregard (we did).
2) There is a dungeon adventure, in the "five room dungeon" format (years and years before that was coined). Unfortunately, the combat encounters herein are pretty lame - hobgoblins, kobolds, and zombies, with very little tying them together.
3) The murder mystery angle is nice, and pretty open-ended. The city rioting as the investigation continues is a nice touch. Plus, unlike many adventures of the time, it doesn't assume on the side the PCs take - they could side with or against the rioters. Great fun! (I don't think too many PCs would side with the guards, for some reason)
4) There is a chase scene, that I don't remember AT ALL. I probably took a look at it, figured it was too rules-heavy, and left it out. I was always like that as a GM.
5) The Veiled Society HQ was kind of a lame finale for the adventure. Another "five room dungeon", which is good, but it lacked "oomph". Just a bunch of low-level human fighters and clerics, and a "Sirenflower" (or, as I like to call 'em, TRIFFIDS).
6) The small-scale encounter maps are a nice touch, and would work well in a 3e game. In a 4e game, you'd probably do well to expand upon them a bit, and maybe add in some terrain effects. But still, a nice starting place.
7) the adventure has some paper models that can be taped together to make nice mini accessories. Never used them for the adventure - we firmly believed that minis were an impediment back in the day. However, I did cut out the cardboard character models and the paper buildings and make them - I have hazy memories of doing so.
The whole adventure is probably 15 pages long. It's not big. It didn't take us weeks of play, like Isle of Dread did. Back in day, we'd play in 8 hour sessions, often saturday and sunday. I think this adventure lasted us one of those 8 hour sessions, even though it was expanded upon with random encounters (which I highly recommend). the game says it will take many nights to finish this adventure. Maybe if you game three hours a night, it will. But if you run it as written, using BECMI, I'd say you should finish it in around 8 hours.
A big plus is that the main city of the adventure, Specularum, isn't deeply detailed, and this adventure could be dropped into any campaign city with little difficulty (especially considering your request, I take this as a plus). The only real setting assumptions it makes are standard D&D - there is a thieves' guild, wizards are not all that uncommon, and people can just approach adventurers and ask for help.
You may have a bit of difficulty converting monsters from BECMI to 3.5E, particularly the numbers. Remember that 12 orcs in BECMI isn't 12 3.5E Orcs, for example. Also remember that 3.5E characters, even core-rule only, are going to have many more options available to them than BECMI characters of a comparable level.
Also, the Veiled Society at the end is TOUGH, just by looking over the notes. The adventure is written for, say, 5 characters of level 1-3. The final humans are all around 3rd level, on average. And the BBEG is a 6th level fighter, with a 3rd Level Fighter helping him out. In other words, expect the group to have some problems.
Funnily enough, for an adventure based around a thieves' guild, I don't think there's a single trap. Or a locked door. Or a trapped chest. Or anything else that will make having a thief handy a good thing. Which is a shame - if you convert it, I'd say you should throw a few traps in.