I'm 5'11" and 200+ lbs. I'm pretty active and I ride my Brompton pretty hard. Plus I carry lots of stuff most of the time. I'm pretty sure a Brompton will fit and serve you fine. I have the extended seat post and with you being only 2" taller than me, I suspect you'll be quite happy. Besides, "Peter" in the NYCE videos is, what, 6'3"? If he can ride a Brompton, I'm sure you can!
As for the LBS being not too keen... I suspect that with Brompton filling such a unique niche, bike shops that carry them are much more keen to sell you stock on hand. Unlike more conventional bikes, if you don't buy a Brompton, the next person won't necessarily either!