Hey Rodge,
Answers inline
I read you can't change a SSO Basic installation to a multisite or high availibility cluster, so you need make the right decision during initial setup.
Correct as changing the installation type later would require a reinstallation of SSO in the current scheme of things
I was wondering how to deal with the vCenter SSO model. Is it easy to deactivate SSO basic installation on a vCenter Server afterwards, and implement an other vCenter SSO model (High Availibility Cluster model to gain more speed)?
Well it's not a simple process, but can be done. Essentially SSO has to be reinstalled and the following KB shows how to re-register the various components back to SSO
http://kb.vmware.com/kb/2033620
Or is it wise to choose the High Availibility Cluster model right away and begin with one node (also involve the loadbalancer at this moment) and if needed you add a new node. Is that possible?
Sure it is possible, but given the convoluted way of getting things to start talking to each other after reinstallation, question is, isn't it wiser to plan the installation during first install 
Regards
a