IMO, the 5D2 is the route. And the good news is that you already own one.
You don't really need the 7D having already a 5D2 unless as a B camera. The 5D3 isn't bringing any revolutionary improvement and if higher isos isn't your concern (today),
the cost of going 5D3 would be better used on a dedicated motion camera.
The 5D2 is hackable, like the GH2, and many productions have been using it. You need to control moiré but the banding issue is quite well controled on the Canon.
Whatever camera you'll use, shoot manual focus and if you can dedicated motion lenses. On the other hand, if you choose to use cinema lenses (do not mean C-monut here),
the 7D is a better choice than the 5D2. Setting a 7D to PL is easier than a 5D2.
The best compromised in sensor format for video IMHO is the m4/3 mirrorless standart.
But I can only recommend the GH2 with some reservations. It's a camera capable of smoking a 5D2, but it's delicate to set-up and in use requires a lot of fine-touch because the DR is more limited, and
because it has a banding issue way more pronunced than the Canons. But the mount is much more interesting and gives access to a wider range of profesional lenses.
The latest hacks are really good, I could get the banding under control and they are developping new promissing hacks currently.
On the video, you have, without breaking the bank account, 2 cameras.
-One is the Panasonic AF100: Haleluia, it features an in-built ND filter ! (that should be an obligation). IQ isn't as good as a hack GH2 but on the good side. Not good in higher isos.
-the other one is the Sony FS100: delicate highlights, pink skin tones and no ND (that should be an obligation II), but overall a serious camera and very good in higher isos.
With breaking the bank account but not yet in bankerupcy (you can keep your Chevrolet Corvette): the RED Scarlet. More or less the cost of an antique MF system. But: no 8bits issues and peace of mind. Raw video, 4K, not stellar in extreme isos but you don't need them as you wrote. Great look output, easy workflow but need power, computer power and storage. (in fact not that much more than with a hack GH2...)
Now...all that concerns the image. But the black cat is not that much the image. It's the audio. If you can, hire a tech.
Tripod is KEY !! No mess with tripod. Better you keep the Canon, and put the money on a serious tripod. Think about 2000-3000 euros new as minimum. We're talking about the cost of a still camera. My fav brand is Vitten.
Then the DSLRs...the problem if you shoot with heavy cine-still long lenses is that on the dslrs designs, the contact between the camera and the plate is minimum AND the mount tolerance and robustness aren't good enough. Very bad for filming indeed. It means that from 130ish mm you'll have vibes. It's to the point that you'd need a special rig config to secure the lens itself, such rig system is to the Arri standart and aren't cheap at all. In short, the camera+lens has to be a one solid peice of block. You can see that in this movie (with the marines) they filmed with the 5D2, when used long focal there is an assistant that just secure the lens with his hand to avoid vibrations, despite the lens being attached to the rig, otherwise it's impossible. I had this issue and it's not fun and the solutions are not cheap.
I'd keep the 5D2, completly forgot the 5Departmentmarketing3 and put this money in: A pro grade rig system and a professional video tripod.
Then, take your old FD manual lenses and start to enjoy.
About rigs. The very important thing is to have the most compact config you can. You really need to feel that the all system is a compact block, and most rig I'm seeing aren't well designed and the handlers badly placec etc...except the expensive stuff. Avoid like the plague a configuration that looks messy and too large. The rig has to stick to the camera+lenses and the all system has to be balanced, almost as steady. You want to have one handler at that point. Avoid the handlers too lower placed if you focus. Ideally they should be placed on the side of the camera and not under and with an angle. (unless you have an assistant that pulls focus and there it's more flexible).
Here again, the affordable is generally not working. I've been trying them and no, it doesn't work the way it should. Don't hesitate to spend money on that too. More than the camera itself. Take care a lot of all the screws, cablery, all has to be military grade in motion or you'd have issues at one point or another. No plastic cheap stuff by any means, no fragile components like mini hdmi.
If you look at the cage of the Scarlet, IMO that's really a superb config. Red understands the motion needs, DSLRs manufacturers don't give a damn. They are just playing with a tech that costs nothing to them and a myriad of third-party accessories have been flourished all over the world, most of wich aren't very good and you'd through money away.
Try to be the most simple you can in everything, included post prod.
Considering all I wrote, I'd choose tho shoot the 5D2 with a serious rig, and the cost in this case will be the rig set-up or directly go for one of the dedicated cameras and the cost in this case will be the all system.
With the dedicated camera you'll gain in: handling (rig being an additional gear), moire, rolling shutter, pro connections (XLR), EVF, more surface contact on plate, and in one camera ND built-in.