Here goes my first post! =) Sorry if it's a lot more than you're asking for! lol
Note: I've never shot a 40D or 50D, but you should be able to compare to my experiences from T2i to 5D!
I picked up a 5D classic less than 6 months ago, and I can definitely say that it is STILL a great camera. So good, in fact, that I picked up a second one about two months ago. Two spectacular FF bodies (with some investment in lenses) for cheaper than one 5D2 or 6D.
My first camera was a T2i, and I can say without a single doubt that the 5Dc outperforms the T2i in all of the most important categories for stills. My T2i has literally been shelved since I got the first 5Dc (That says a lot, considering that I have some crazy good EF-S glass), and it only comes out when I need to do some multi-camera video work.
There's some sort of magic to the files created by the 5D classic. Seriously, I could not believe how crisp, sharp, rich, etc. the files coming out of the sensor in the 5D were, especially in comparison to anything I had shot on crop sensors before. The 12 megapixels are more than enough when you're nailing the focus, and for the sensor size, they give this great sense of depth and tonality (especially skin). The RAW files produced are only about 12MB each, so they are easy to manage and retain scary-good IQ.
Comparing the AF between my T2i and 5Dc, I'd say the 5Dc might have a slight edge in AF performance. It could be that I'm using lenses with better AF in general? I use the 85mm 1.8 with USM frequently, and it focuses fast. I can't complain and it does action well from my experience. I've also tried a few other USM, HSM, and USD (specifically the new Tamron 24-70) lenses, and they all work well enough for me to never complain. I shoot a lot of nightclub events with the 5Dc and the pretty new 40mm 2.8 STM, and I can say that the combo has been really good in terms of AF in the low light (not the fastest ever, but very good and accurate) and has no problems keeping up when I use an assist beam from my flash. Keep in mind I only use the middle point with all of these lenses, however. I think you would have to be doing some extreme stuff to stump the 5Dc's middle point, and of the 7000 or so clicks I've taken in the last few months, it's rare to find missed focus that was the camera's fault--actually, not sure if you can yet! It's super reliable in that center point.
The viewfinder in the 5Dc is several leagues beyond that of the T2i. I was actually super shocked when I went back to my T2i after shooting the 5Dc exclusively for a few months to learn it inside and out. When I went back, I felt claustrophobic! I didn't know how I was composing with such a small viewfinder, let alone manually focusing old M42 lenses with it!
On the 5D, manually focusing via viewfinder is noticeably easier to do, but I understand that it's not truly great until you swap out the focusing screen, which I won't be doing until my next paycheck lol. If you have live view, 10X focusing is easier and more accurate when your target is easy to track and you have some sort of stabilization, so that's probably the only thing I really miss from my T2i, but it's really easy to live without lol.
The ISO capabilities are more than enough on the 5D, and it's super clean up through 1000/1250, very useable at 1600, and good enough at H 3200 if you really need it. This is another department where the 5D stomps out my T2i. I lived in fear of anything above 400 ISO, especially since the T2i only makes full-stop jumps in ISO unless if you have Magic Lantern installed. With ML on my T2i, I would stop at 640 ISO (ML lets you get the intermediate ISO's, which was huge for me on my T2i!) and only use 800-1250 if absolutely necessary, and 1600 for emergencies, 3200 if I needed the shot if my life depended on it, and 6400+ never. I'm exaggerating a little, but I honestly have no fear of using 1250 or 1600 on the 5D when I need it, whereas my images were falling apart at that point on the T2i. The only downside to 5D ISO is the fact that you can't use auto-ISO like you can with the T2i, but I never did anyway. Not sure how how the ISO performance stacks up compared to the 40D, 50D, 60D, or 7D, but for me personally, shooting above 1250 isn't really a priority, as I've learned how to stay below that for about 4 years now! I'd probably be spoiled in ISO if I shot on a 5D3 or 1DX, and even now I try not to get addicted to high ISO when I shoot on my OM-D lol.
The 3 FPS isn't so impressive with the 5Dc, so it's probably not the best option for fast-paced sports when you are shooting professionally. Honestly, it's good enough for me and my shooting style, but if I were paying all of my bills and rent with photography, I would invest in more FPS elsewhere. In your case, the 50D should be more than enough when you really need it. So far, I've really only shot in single mode, and I've had no problems just timing everything and nailing the photo.
And now for the biggest letdown of the 5D classic: THE SCREEN ON THE BACK SUCKS! Truth be told, it's the first thing you will notice, and the biggest turn-off when you use the camera for the first few weeks. I thought it was disgusting since I was used to a relatively nice and sharp one on my T2i. The 5Dc's screen is pretty small, very low res, and it has a green cast over the blacks for some reason. I don't know how this compares to the 40D or 50D screens, but chances are that the 5D's will suck in comparison. lol I felt as though the 5D sucked when I saw the images from the back, and I was scared that I had wasted money on the camera instead of a decent lens. When I pulled up the frames on my laptop after a day of shooting and disappointment, I was completely blown away. I wanted to cry out of joy. That's how big of a difference it was from crappy back screen to computer lol. At the moment, I'm used to the screen and do not mind it at all anymore, looking at it only for exposure and composition for the first few frames of a shoot.
OVERALL, I'd say make the jump, you won't regret it. Get the best condition 5D classic you possibly can, get it serviced if you really need to, and it will likely stick with you for a long time, especially since you've got some really good glass to use with it right off the bat! The camera is super reliable and produces beautiful images after you get past the little quirks. Even against the more modern cameras it will hold its own, and you might not need to upgrade again for a while unless if you really need video, high fps, extreme ISOs, or elite autofocus.