Yeah, you definitely want to get started if you haven't already. So no reason to miss out on any of the cool features of the better packs in the mean time only to need to upgrade later anyway.
Ultimate + question bank makes it worth it. OP, if you can eventually afford to upgrade, do so. You get so many extra problem sets, solutions to every PT, and, again, the amazing question bank.
... is a MAJOR reason to upgrade. Having every PT and the ... feature you get when you upgrade. And of course, the extra ... decide though! You can always upgrade anytime !
I'm sure you can find a Cambridge packet somewhere on eBay/amazon, but they will most likely be pricey since the company no longer sells them. If you upgrade to ultimate+ though you can get all questions with explanations!
I too have this same question. I am about 1/3 of the way through CC. I know I will eventually upgrade just unsure if I should do it once I finish the core or at a different time.