I have no experience with InterServer hosting, but I will suggest you to start with month-to-month billing period with any new hosting provider. When you are satisfied and happy, change the billing period according to your budget and discounts.
Better be safe than sorry.
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Check out how long they take to respond. Besides speed of responses, check to see if they are technically competent. You wouldn't want to sign up for a host that is run by a bunch of salesmen who only know how to sell and not fix problems.