HD partitions, keychain, filevault, time machine - coordinated setup

Question

Let me start by saying this question covers numerous individual components of OS X and how to set them up in a manner which makes sense. Thus I hope this is in the correct forum. This could be a very long question and perhaps beyond this forum. In this case, being directed to an additional online resource would also be helpful and appreciated.
I am new to Mac and as yet have not started using keychain or filevault, but want to in the near future. As such, I don't really know what questions I need to ask to get it all setup the best way. Since the iMac is the main family computer it has four user accounts set up on it, only one being an administrator. I want to ensure that TM, KC, and FV, plus the partitions and user accounts, plus the MBP, are set up in a coordinated way.
I will lay it all out:
I have an iMac and a MBP, plus a new external HD and I want to start using it as a backup. I want to use Time Machine for this. I will need to upgrade my iMac to Leopard from Tiger but I was planning on doing that anyway.
So once I have upgraded the OS, here are some of the questions which come to mind before I start:
1) How do I set up the external HD so each user can only access their backup files. Obviously this isn't a problem on the iMac's internal HD but how does this translate when you go external?
2) Does it need to be partitioned in order to do this? Are there any other advantages to partitioning it anyway, it is 1TB?
3) How does setting up secure individual user account access impact TM? Can I just set up TM within each account to do scheduled backups of each users data?
4) Does adding a second machine, the MBP, change any of the above? It already has Leopard on it but isn't used as the "main" machine. It too has different accounts etc. much like the iMac
Perhaps I'm making this more complicated than it needs to be but I just want to make sure I don't set something up at the begnning which forces me down a road I don't want to go down later on.
Hopefully this makes sense and thank you very much for any pointers.

Answer 1

you have some misunderstanding about TM. it only need to be set up once per computer and it will back up the whole computer including all users. unix permssions will prevent one user from accessing TM backups of another user and you don't have to do anything at all about that one.
If you want to use one TM drive with two computers you have two options.
1. you can constantly switch the drive between two computers as you can not connect an external to two computers at once. This is a major pain in my opinion.
2. you can get a Time Capsule and back up both computers wirelessly to that.
Lastly let me urge not to use filevault unless you absolutely must. filevault can cause all kinds of issues and doesn't play nicely with many things in aprticular Time machine. you can't back up a filevaulted account to TM while you are logged in and you can't use TM to retrieve individual files from a TM backup of a filevaulted account while you are logged in.