Welcome Andrew, you've certainly come to the right place for advise etc.
Definotely catch up with the NSW gang, definotely getting along to the local Cobra Club meetings if you can -
http://www.cobracarclubnsw.org.au/ Cobra ownership is definitely not a requirement and it's very handy to join before you buy the Cobra.
And if you haven't been before, make sure you get along to the Cobra Nationals at Wakefield Park in October.
$100k is a pretty good budget, you can certainly buy/get built a new turnkey Cobra for that money.
Personally I wouldn't be concerned about your skill level, I would aim to build the car yourself, you will pick up the required knowledge pretty quickly if you do your research and put the effort in. Plus you can always outsource any work you are not comfortable with to the experts or get expert advice.
Hell nothing wrong with Focus XR5. Very nice little unit actually.