1) If you have 10 different SIP Lines/Accounts you'll have to use the Enterprise version since the next lincense down only supports 3 lines.
If you have 1 SIP account that supports 10 concurrent calls/lines, you should only need a Small Interactive System license
2) On the sales page, it will tell you if there are currently any discounts/sale prices being offered. The discounts are a limited time offer only.