a fine violin will have a soothing sound, and some also have mellow (darker) sound. once you heard a fine violin, you'll fall in love.

but that's just my assumption, i've come across many people took up viola because they think violin has sound that's too bright.

also take note that, viola need greater bow pressure (and generally much effort to play than violin) and viola is of course much heavier than violin.

you have your own personal preference so don't get affected by my comments above, it's all up to you to decide.
1) empire might have some orchestra player's contact numbers under his sleeve.
2) for viola, better get from orchestra players or your teacher. most shops don't carry better student violas, many are pretty cheap (you know, you get what you pay for). but you can sometimes find some gems from orchestra players where they have older viola selling for cheap but sounds good.
3) generally, yes. but when you're looking 5 figures and above, not necessary.
4) there's no fixed full size for viola. smallest full size viola is 15", up to 17" which is for giants.