G’day mate! Judging by your opening line you’ll fit right in! Welcome to the fun house!
The Thinline kits are nice, and if that’s what you want to build, then go for it. We’ve had folks do jazz boxes as their first builds and had them turn out beautifully. I don’t see a Thinline should be any different.
I will need to leave it up to those that have built that kit to advise you on the P90, I’ve never fitted one so I can’t comment.