I'll try to give you a good picture of what you are asking for.
Good start to jump into 9. It's going to the most stable and I think you'll like the UI changes. I'm trying to drive to get out environment up to 9. (I don't do the installs.) BTW, I believe you need to upgrade your Domino servers to 9 as well, depending on your entitlement (complete. communicate, conference).
1. The SSC is the first thing you need. You will need DB2 to support it and using DB2, you can change your meetings to the new style (sounds like you continued to support "classic" meeting rooms under your 8.5.) Not knowing your use community (number of uses) I can't say with 100% certainty that you need ST Mux servers, depends on how far you want to go with redundancy and fail over. The ST Proxy will be needed for the Meeting server support. The Proxy can also provide extensibility to other areas of your intranet for presence/awareness status.
2. Not knowing the user base size, I can't recommend a number of servers. The SSC, ST Proxy & DB2 can reside on the same Websphere server. In the end you will have two DB2 database, one for SSC config data and the other for Meeting room data.
3. You will need a MM server for A/V. ST 9 changed the way A/V was installed and without user size specs, I can't say. There are bandwidth managers and TURN servers, so depending on how you wish to deploy A/V, I'm not certain the minimum you will need. I'm not as familar with the new 9 A/V components. Plan on a dedicated server for A/V either way.
4. The ST8.5.x files you need minimally is the VPUserInfo.nsf. I would strongly suggest setting up on new vitual servers in paralle to your 8.5 environment. Assign DNS names according to roles for the server, ie. comm9.mycompany.com, mtgs9.mycompany.com, ect. Then when you are ready to switch, use update the alias to the 9 servers. (Hoping that you didn't have release numbers in the main DNS names.)
5. LDAP is required. I'm surprised you made it through 8.5 without one. The LDAP can be Domino LDAP, Active Directory LDAP, V3-compliant LDAP servers.
There is a lot of mobile support for chat and meetings. The installation package have the android, iOS and Blackberry devices for Sametime. Sounds like a fun project, wish I could help.
The Wiki provides more details: http://www-10.lotus.com/ldd/stwiki.nsf/xpDocViewer.xsp?lookupName=Administering+Sametime+9.0+documentation#action=openDocument&res_title=Migrating_from_Sametime_8.5.2x_st9&content=pdcontent
Hope this helps