Re your point 2.
a) ouwiki does not provide upgrade facility yet. This will be added if it is incorporated into core.
b) the new wiki question is for Moodle 2.0, not 1.9. You won't have any issues with 1.9 upgrade (well except the current wiki will still be buggy and rubbish
c) there is no how-to for installing ouwiki, but here are the instructions:
0) make sure you have php5
1) download the ouwiki plugin from moodle.org and put it in mod/ouwiki
2) if you have the 'mbstring' php extension enabled (we just found out this is required, may need to change code at some point so it isn't), download the ousearch plugin from moodle.org and put it in blocks/ousearch
3) visit admin page
Whichever wiki is included in moodle 2, I think you can be assured that the install and upgrade process from ewiki to nwiki (or ouwiki, in the unlikely case, etc) will be completely automatic and that any requirements such as required php version, extensions will be checked for you on the 'green lights' screen.