Guidance wanted: 650b Randonneurs

Ok, so I am looking for my next bike, which I already know is going to be fixed. After a lot of thinking about this, I have pretty much decided on 650b wheels for a frame made for them, as opposed to slapping them on a frame for 700c.

New, boutique frame makers aside, (and I am aware of the VO Polyvalent), are there any retro steel (obviously) frames that I should consider hunting for? Anyone been down this route before?

Obviously vertical drops are a no no unless I go for an eno hub. Haven't quite decided there yet. Even something complete that currently has a geared hub; least that sorts my chainline measurement out.

So far I have a rough image of:

60cm 650b frame with forks.
Low flange track hubs, 36 spokes.
GB mudguards, though may end up with SKS, can't decide yet.
Randonneur bars.
Obligatory Brooks.
Fluted crank, (or if we are looking at a period machine, steel would be fine if chainring ok and I can achieve 69 GI with suitable track cog)

Any advice, at all, would be appreciated.

For me a 650b randonneur has to be French, although the prices can be extravagant to say the least. Still if you avoid the big named collectibles, Rene Herse and Alex Singer, there are some fine bikes that can be had at a fraction of the cost. I have seen bikes by LeJune and Urago sell for very reasonable prices, less than $500 usd, as a complete bike. There are a handful of Ebay sellers that regularly list these bikes and frames, rhclassics is a good place to start although they tend to offer high end stuff.

I would normally agree with you about being French frames, but part of me is reluctant because if trying to get chainline correct for fixed, I am going to open myself out to a world of hurt with BB choice.

Maybe. I am willing to be corrected on this...

Finding the right French thread bottom bracket may take some time but they are not too rare. Also I am not aware of any chain line limitations with fixed gear hubs and French BBs