The owner makes 50% of the dog. If you know what you are doing (like my wife, not me) you can have the most friendly and amazing chihuahuas as opposed to those terrors grandmothers raise.
Be firm, stick to routine, you are the boss and just because the dog is physically small doesn't mean it needs to be treated like a puppy, carried around everywhere, sleep on the bed etc.
Feed them high quality food (Royal Canin for example [/queue the opinions...don't care]), stick the portions, don't feed them your left overs and make sure you take them to the vet for their checkups once a year and you'll have a healthy dog.
We've never needed pet insurance - it's a money pit imo.
EDIT: don't get chihuahuas, that's not what I meant