If it were me, I'd have a really good play with everything you've chosen first. That will show you where you need to go next.
Don't for get things like passive attenuators for calming down LFO's for modulation.
I'm fairly sure you could live without the Output module and the Scope until later in your journey.
I'd really like a scope but I do't want a small one and I don't want to pay for a big one, I'll probably buy an Ebay Oscilloscope cheap at some point and hook that up outside the case.
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.