I think you have to draft a potentially great QB whenever you have the opportunity to do so. You can't wait until you are ready and everything is convenient to draft a QB, because there's always the chance that there's no QB available to you when you need him.
I think as far as team building goes. I think if you don't have a QB, you probably should focus on staying sort of lean up until you get a QB and then go all in. I think it makes the most amount of sense to focus on building the offense first, such that you have the environment for the QB to succeed in place when they arrive, and let the defense wait a little bit and then toss FA money at the defense once you know you are in the window.
In general, I think you should let your best QB start, and then when your young QB is capable of beating out your current starter, that's when you make the switch. Then bring in a new young QB to push the current starter. When the time comes, trade one of the QBs for draft capital and draft a new guy. I think you probably need to have 3 guys on the roster to make this cycle work.
I'd also probably employ older veteran QBs that aren't on the roster to run the scout team. Let the young players focus on getting better, not being fodder for the starters.