What differentiates a high quality overcoat from a low quality overcoat
i had two overcoats made last winter which are still serving me very well. full canvas is very important!
yes high quality overcoats are indeed canvassed and shouldn't be fused (unless the fabric weight is extremely light, in which case fusing may be required).
it does go over the suit, but still needs to fit well and since it will be canvassed, the overcoat will conform to your body better than a fused suit.
also the lining and fabric play a big role. the cheaper the lining used, the less likely the overcoat will stand the test of time.
hope this helps.
Once you've spent the extra money on the material construction costs aren't going to be a big deal so you don't skimp here either.
But even a cheaper coat will last almost forever.