Set-up - Buy from a reputable brick and mortor store rather than the internet. They can make sure your instrument is properly adjusted, and keep it sounding as good as possible. A good setup is vital for any violin, both for playability and tone. The quality and fitting of the bridge, and especially the fit and adjustment of the sound post make a huge difference in how a violin sounds. Setup is so, SO important... and I love an instrument that has a history... warm, big tone... and is set up easily playable.
Sound - I think violin sound and the ability to color it is part of the addiction to get us to keep playing! Be leery of buying a violin without trying it out first. A well setup and responsive good sounding violin will make you progress a lot faster as you'll enjoy practicing and be motivated since you actually sound good. As it is always a joy to drive a responsive car, it's the same with a violin and bow. If you can get good sound, handling, and feel, then you'll really enjoy it a lot more.
Longevity - I'd look at what you would be willing to spend on something that will carry you through many many years of playing. Spend some money to get the best… A good, responsive violin (and bow) make practicing much more enjoyable, and you'll progress faster. A few hundred dollars' difference in price, amortized over all the hours you're going to be playing, gets pretty insignificant in the long run. quoted from Violinist.com