Although it is really worth investing in a good quality one, even an inexpensive grey or black coat can look really stylish, and can be worn with everything from skinny jeans and ankle boots to a dress and ballet flats. A perfect way of adding interest and making a budget buy more unique is to have the buttons changed - I often find that the only give-away with many decent-quality high-street pieces are their buttons; change them, and you have a truly magnificent find with a bit of an edge.
If you are interested in vintage, it can be a great way of finding a very high-quality, wool jacket or pea coat for a fraction of the price of a new one made from the same fabric. The camel coat is a true classic, and for a good reason; the neutral colour goes with everything and somehow adds a touch of elegance to any outfit. I personally love to add a simple silk scarf in a contrasting colour, and then I'm good to go.
Not forgetting to mention the all important fell-good factor that great style can impart, dressing in the right coat can, I believe, give off an aura of confident sophistication (in the best sense) which can be helpful in actually making you feel more confident. I often live by the mantra "fake it 'til you make it",