I to am just starting to get into overcoats, although I already have a custom made brown cashmere coat that comes down to my knees. From my understanding (after speaking with 2 SF members who know a lot about overcoats, upr_cust) I have found out that the most classic and the biggest staple overcoat to have is a navy knee-length overcoat. Depending on what you get, I seem to think that a single breasted overcoat is a lot sleeker and slightly more formal looking then a DB overcoat. Obviously that's a personal preference and will change based upon the coat.
The questions you need to ask yourself are:
1.) do you want more of a modern (younger man) look?
if so, I'd suggest getting a coat the comes down to just above your knee.
2.) Do you plan on wear a suit jacket under it most of the time?
It seems like you will be wearing a jacket under it so you want the right fit. For fit you want to buy the coat in your typical suit size and have it tailored to fit snuggly in the body when there is a jacket under it. I'd suggest wearing a suit while shopping for it and having it tailored. You also should expect some "drape" in the jacket from the waist down to be able to walk and move in it.
Finally, to answer your last question.
Waist length is not generally supposed to be worn with a suit underneath it. Typically you'll see people wearing classic pea coats in waist length. The Pros of the waist length are those jackets are typically more casual, they are easier carry (if you get to hot and don't need it) etc. The casualness of the coat can also be considered a con.
The full length coat cons are since it is long and warm you can really only wear them when it is colder out, its more of a 1 season (winter) coat. The pros are you can dress them up and wear them formally and you can even dress them down slightly and wear them more causally. A navy single breasted cashmere coat is very versatile and will probably not go out of style anytime soon.
I hope this helped.