Hi, jss:
As far as I can see, use of the SPACE key (largest key on the keyboard) to start and stop playback is pretty much industry standard. There are exceptions, but those are usually "extreme" DAWs which go out of their way to look and feel different from the rest.
Entering Select mode is currently done with the ENTER key, which I don't see as any more or less convenient than the ESC key, once you know which key to push. Either the standard Enter/Return key or the Enter key on the numeric pad can be used, so the ESC key's separation from other keys is pretty much offset by the numeric ENTER key's position on the opposite corner of the keyboard.
I don't use Sibelius very often, but SPACE is the playback key there. Checking the Sibelius shortcuts list, ESCAPE is used to "unload arrow", which I assume is to deselect.
Notation Composer does not currently act as a VST host, but you can easily work around that with a virtual midi cable and any number of freeware VST hosts or other sound sources. In fact, for some VST hosts, you won't need a virtual midi cable. Many here use Synthfont; I have CoolSoft's Virtual Midi Synth installed for quick and dirty playback/recording. For final stuff, I use some or all of the ridiculous number of hardware synths and samplers I have hooked up to my system.
Two tutorials on that subject can be found here:
http://www.notation.com/vb-forum/showthread.php?t=3863
and here:
http://www.notation.com/vb-forum/showthread.php?t=3864
Hope that helps.
David