it really is down to how much you want to pay and what sort of impact you want in the garden. I usually buy small plants to enjoy them growing. my brother will buy big plants for his small garden so it is 'mature' and give immediate satisfaction.

Depends on how much money you have and how much of a rush you are in for a decent clump. You don't say where you are so I can't advise on spreading time. Faster in the Scilly Isles than at John o'Groats