I haven't heard of an iMac where you cannot change RAM yourself. A one may exist, and I may have not noticed it, but the general rule of thumb is that you can change at least RAM yourself within minutes, and in some cases HDD, too. The MacBook Air is of course an exception, and in some cases, like with the latest mini, changing the HDD is a bit tricky. Doable, but tricky.
My suggestion would be just to buy a stock iMac which you can afford with some change, then buy a matching pair of 3rd party RAM. Not that hard to find. Just make sure you always use a matching pair of RAM modules, if both banks are in use.
That will save you enough money to buy an external HDD. Or, go and buy a fast, and the biggest one you can afford. You know, for your FCPX projects, photos and stuff. That way the size of the internal HDD of the iMac is not that critical.
That's about it.
If I was the owner of this forum, I would make sure not to touch that particular topic. Not that there's anything wrong in asking or discussing the topic, but don't be surprised if the practical suggestions won't come from the forum owners or even moderators. Instructing where and how to abuse a software license might be on the hazy side of legally and morally smart things to do. Sure it's harmless to the normal punters, but for forum supervisors it's another matter. Potentially, at least.
I'd say a hackintosh may be an ego booster or a political statement of a sort for some computer nerds, but for any practical use and for work it wouldn't be worth the hassle. Just get the real thing, new or used, and then max out the memory yourself.