Please post in the right section. This is for Video over IP questions, not general upgrade questions.
If you want to stick with Unity you need 5.0(1) as minimum. Otherwise you can migrate to CUC 8.X as you mention.
SCCP Compatibility Matrix: Cisco Unity, the Cisco Unity-CM TSP, Cisco Unified CM, and Cisco Unified CM Express
http://www.cisco.com/en/US/partner/docs/voice_ip_comm/unity/compatibility/matrix/cutspmtx.html
TAC is a break and fix organization, they don't provide prices.
You need to either reach pre-sales or a reseller for prices and a BOM.
Either middle version is fine, AFAIK there are already some offers about the upgrade like 3x3, again, this is something you need to discuss with pre-sales a or a reseller. But you don't need to buy the 7.X licence unless you plan to start the ccm service in that version.
HTH
java
If this helps, please rate
www.cisco.com/go/pdihelpdesk
HTH
java
if this helps, please rate