Single DB Engine and multiple DB Instances approach

Hi Tech Arch gurus,
Need some advice from all of you. We are currently planning to install SAP  training systems to latest versions.
We are getting recommendations to goahead with Dedicated host for Oracle Installation where we have Single DB Engine and multiple DB Instances for all different 7 SAP Servers (ECC6, CRM7, BI/NW7.3, SOLMAN7.01, SRM7, EP/NW7.3, PI7.1).
Certain things that need to be clarified before we go ahead with this:
1.     How complex/simple is this approach ?
2.     If there are some issues noticed in the future (Performance issues etc) in this landscape , how much effort will be required to migrate out a DB instance for an SAP product to another machine. We would be having both ABAP and Java stack for our systems.
3.     We would later plan for adding some more systems into the landscape like SAP MDM, SAP CE, SAP BPC etc. If we add all these systems with a single DB engine, would there be performance issues etc. (We are operating under assumption that underlying hardware of the DB engine will be powerful enough)
Can you please provide some expert guidance on this approach?
Your response would be much appreciated.
Thanks,
MD

As a consultant I never dared to do different from what SAP says in guides or notes, at least when it comes to PRD systems.
For QA and Test Systems, discussions might be needed with the customer, depending on how important these systems are,
and for sandboxes I already did some stuff, that will even scare some Oracle guys
As Stefan says, the technical setup is not that complicate at all.
But I never had to fight for a support call in a non-supported Environment, so I can't say.
I always decided to be on the safe side.
If you have your Oracle License directly from Oracle you might disscuss such special support issues with these guys.
The problem is: If you get a call, and the first level support guy from SAP spots the non-supported environment, it MIGHT be possible that you need to escalate and may be pay for the support, but I suppose they will not let your system die
May be some of the SAP moderator guys can throw a comment in on this.
Volker

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