If you're new to Sametime why bother? Just start with the latest version ;-)
Anyway...you're going to deploy the limited use client basically means that you'll use the built-in IM capabilities in Lotus Notes instead of a full Sametime client? If so, then you won't get Web conferencing among other things. That's too bad because this could be a very effective tool to help you save time and €€€.
Here's a table describing the differences:
Here's how to set things up:
If you want to know the difference between Sametime 7.5 and 8.0.x check out the "New in this release" section in the release notes: http://www.ibm.com/developerworks/lotus/documentation/sametime/