My own philosophy on overcoats is as follows:
As an "over" coat, it's going over several layers of other clothing. As a result, it's never going to be as fitted as a suit; nor should it be. I wouldn't ever get one so fitted that I could only wear certain clothing underneath - I like to wear mine both over suits, and over jeans with a bulky sweater.
As for construction, overcoats follow the same basic pattern as suits - full canvas hand-work is the best, all the way down.
Here's where my philosophy becomes contradictory though: Because overcoats aren't as fitted, you can cheat on the fitting process (or just go OTR) and still look quite good, and so you should therefore save your money for a place (suit, shoes) where it will make more of a difference. On the other hand, a good overcoat can last you 10, 15, or 20 years, and during the winter may be something that sees daily wear. For these reasons, it's worth spending a bit more on an overcoat.
So where does that leave me?
Well my 10 year old, but fantastic OTR overcoat is on its last legs (if it's still standing at all). So now I have to choose whether to find a bargain somewhere, or pay a bit more for a M2M one from Mr. Ned, or pay a LOT more and find something that's full canvass and hand-made with the idea of it lasting 20 years.
Not sure if any of that helped, but it is what it is....