This is enormously helpful! Thank you all for your input, I have looked into Gunnar Sport and the Jamis bikes now...I have to say I'm hoping to find a lugged bike (restricts options even more I know).
While I love to research and learn about bikes I prefer to ride them so I am new to this "frame geometry" stuff, but it is really time I learn . By "endurance geometry" does that mean I am looking for more of a touring frame? I want to do centuries and maybe longer, even like the idea of very light camping every once in a while but I was told by a custom builder that a touring frame (the Surly Long Haul Trucker was the one we were discussing) would be far too overbuilt and "dead" feeling of a ride for my size (I'm about 120lbs and 5'4"), especially since 99% of the time it would presumably be for joyriding/transport in town.

Looking through Mercian's "frame builder" they have offered as styles "race, sportive, tour & track". I'm sure I would have to tweak it still (Robert that's a GREAT idea!), but what of these styles would assumably be the closest to my needs? Would it be "tour"? Honestly I have no idea what the differences would be between those styles, frame-wise (I know what the finished builds would look like somewhat but don't understand how the styles affect the position of the rider on the frame)?