It is precisely to help you make this decision that we built our insurance comparison facility. Using ivisitorinsurance's comparison engine you can evaluate different plans based on their cost, the deductible and their rating. This will help you identify the plan that best suits for your needs.
You should purchase the insurance only after being certain of your travel plans (having the passport/visa papers and the airline tickets in order). It is safest to start the insurance coverage from the date of departure from your native country.
Yes. You can purchase the insurance coverage on behalf of others in their absence.
Some insurance plans do refund money if given enough advance notice, however since travelers insurance is typically for a short duration, they are often not refundable. If this situation is a concern for you, you should look out for insurance plans which are renewable. Such plans are available among the plans listed in our comparison engine.