Maybe this is too obvious to point out, but I think the biggest influence on string buzz (aside from a neck that just needs truss-rod adjustment) with low action is going to be the player's style. I received my Series II with extremely low action -- so low that the low B string had significant finger buzz (for me) when I picked up the bass. Raise the action a bit and it was better, but I came to realize that the finger buzz problem on that low B string was more my playing style than issues with the neck. I.e., you take a big fat string sitting _very_ close to the fingerboard and strike it hard, and you're going to get some buzz. Think of it like this: the horizontal profile of a string at rest is a flat plane, but a string that is resonating is not a flat plane, it is curved. However, draw a straight line from nut to end of the fingerboard under any string, and you do have a line that sits (at a profile) in a flat plane, i.e. it is not curved. So, lower the action enough, strike the string hard enough, and the string is going to hit the fingerboard (or the frets if you have them).
I spent the first 12 or 13 years playing a less than adequate bass through crappy gear and this taught me to play hard (and get lots of blisters on my fingers) and break bass strings. It also meant, I always had to have a high action. In the last couple years, as I've started to get gear (and now an incredible instrument!!) that gives me way more expression, I'm learning to strike the strings easier and let the lighter playing style clean up what's happening right on the instrument while letting the amplifier do its job too.
I think the downside is, your friend at the guitar shop is probably right. On the upside, I agree with other posters, that if you go in on an Alembic and specify that you are looking for very low action, they'll work with you to get it. Just be aware that your playing style may wreak a little reverse-limbo havoc upon the string buzz problem.
Finally, there's the inevitable setup issue. From one climate to the next, a bass neck is going to move. With this and the rest in mind, it might be worth your peace of mind to consider the possibility of working more closely with folks at Alembic by planning one or two visits to the factory. One to sit down, try some basses and talk action with their setup folks, and another once your bass is built and has been setup. That way you can give the best input, get some insight into manufacturing and setup issues related to action, and finally feel that action before it gets shipped to a completely different climate (where the neck will move some). It's not that far from TX -- I made a trip down to Santa Rosa myself last year all the way from Juneau, Alaska and there's not even a road out of Juneau ;-)