The minimum coverage you need is 30,000 Euros and this must include repatriation (return to your home country).
You can easily purchase this online - this is what I do, and my preferred provider is Axa - simply because they have a straight forward website and the entire process takes less than 10 minutes of one's time.
Start here: https://www.axa-schengen.com/en
Your credit card may also provide you insurance. Make sure it meets the guidelines for Schengen (just ask your card issuer for the insurance papers).
The company letter should state (at minimum):
- Your full, legal name
- Your passport number (or other travel document number)
- Your position in the company
- The date you were employed
- Your total salary
In some countries you specifically need permission to leave the job, in that case it should state the start and end of your leave.
If your company is paying part of your expenses (for example, on a business trip), then it should also be mentioned on the letter.