Renting that lens to get a feel for it is a great idea! It does require some bits to work with standard rod setups and what better way to test than to rent? (Well, unless you have a local pal that owns one). The Allstar lens support certainly gets the job done, I have that one on mine.
Back in the good ole Red One days (Only last year), there was just slightly less dire need to get tons of storage and optimizing our machines for ultimate speed and superfast throughput, but of course, now with 5K a reality and 6K just a few months away, we are again past the threshold of the cutting edge, YES!. I have been trying my best to get away with not getting a Rocket, but after a 30 hour transcode of 2 hours of 2K footage to 1080P, I have rethought that strategy. Mind you, I was only using an i7 laptop with 8GB of ram. That little experience was an eye opener, and the job did go a long way to help pay for a "Battle-tested" Rocket. Now all I need is to build a computer around that card.
There are more ways to get from R3D to deliverables than ever and everyone has a different workflow, these are great times! CS6 and the mercury playback engine is just immense! But with all the different hardware configurations and workflows, the single universal practice shared here, is to test, test and test.
Best of luck, and have a great time!