Are you sure you want to buy this patch of common property or would an exclusive use by-law (that could still involve a payment but wouldn’t require a redistribution of Unit Entitlements) not be a simpler way to go?
In any case, you only require 75 percent of those voting at a general meeting to be in favour so I wouldn’t worry about those who won’t be there – unless someone harvests their proxies specifically to stymie your plans.
The by-law required to either sell you the lot or give you exclusive use of it will have significant legal implications so your next call really should be to an experienced strata lawyer who will also answer your questions about delaying the application for a DA.
By the way, I wouldn’t think that will be an obstacle in itself, provided the Owners Corp is in favour of your plans. But don’t assume their agreement to sell or lease you the CP represents approval to do what you are planning – that should probably be dealt with separately.