I'm thinking about starting to build a single speed or fixie. What are the things I should look for on frame! Braze on or not, 700c wheels or other, etc...

street cred

Owning too many bikes too list means you probably know what you want out of a frame. So what is it? We can help guide you.

I do have lots of bikes but not much of roadies types. I have mostly cruisers and mountains. I know how to size but not sure about material (I guess steelies are best?) or what kind of geometry I should look for. Are the brazed on attachment to run cables desirable on a fixie frame? Would a cheap steelie do the work or should I look more for a higher end frame?

Braze on? You mean brazed joints and not front derailleur, right?

Your being too vague for anyone to give good advice. What kind of riding will you be doing? Nothing is universally "good."