Hi ck25,
Kitesurfing can be expensive when you are starting out, but doesn't need to be and of course once you own it, its free to use. The canny buyer can still get good gear without having to go cheap.
Core Nexus isn't a big air kite. The Core XR is the big air kite. The Nexus is the all rounder, good at everything but not great in any one area. The Nexus however is a slightly more advanced kite and as a newbie, you won't get the most out of the kite without a bit more experience.
We teach on, and sell Duotone Neo's to beginners as they are so plug and play for beginners and experienced riders alike. We have a very good condition 2nd hand, 9m Neo SLS in our store which would be a very good kite for you.
I agree with others, the 142 board is too big for anything other than the first few weeks of solo riding. A 136-138 would be a better choice. Most 90kg riders ride 138 so you can see the 138 would be all you would ever need. I'd say stick with a 136

I used to ride 132 at 88kg but now I ride 138 at 95kg.
DM