I think you might have a point on a slightly longer lens for head shots and wider for bodies.
*see image at bottom, 85mm F1.8 as camera, no cropping etc and dad in place, though slightly too close to the wall (bring him out 1ft).*
After using the 1.8's unless a 2.8 is pinpoint wide open I think I would find it limiting (I tend to shoot at 2.8 when I can) so am thinking primes over zooms here.
Taking a critical look at what I have and after having tried some weight tests....
Weight I think is going to rule out the camera upgrades, both are over 800grams, as I struggle a bit after a while with the 17-55 + 580ex on the 500D I have a feeling I wouldn't manage the load and don't want to be stuck with a 7D or 5D that I can only use on a tripod.
It would be more sensible to try a 50D and see if I can handle the load with a 730gram body, it still has things to offer such as custom modes and better AF.
I guess glass wise my ideal solution would be an 18-135 F1.8 or 2.8 if its super sharp wide open, but neither exist, actually the Tokina 50-135 would be great but I would struggle to lift it.
The 70-300 focal length range is a real nightmare, the faster stuff is too heavy, even the standard IS version may be to heavy, so that just leaves lightweight, and a cut in IQ as I want to be able to hand hold this lens.
The long end, well realistically I wont be able to afford F2.8 and I'm not so sure I need a zoom, I could probably do better by getting a prime, so maybe the 400mm F5.6 is worth a look? its reputedly sharp and as it will be permanently tripod mounted IS isn't an issue, either that or look around for an 300mm F4 secondhand and add a 1.4x extender, I tend not to shoot under F8 outside anyway.
It's just horrible when you know money is finite, and its a once only purchase, so you struggle to make sure you don't make an expensive mistake, something the previous posts have already helped prevent (thanks to all).
Oh the eye problems...glaucoma (can be stopped where it is) and cataracts, the downside to them is I might not be able to be operated on as my other health problems make any surgery a very high risk adventure and they have already indicated they wouldn't want to even think of doing them until I am in my mid to late 60's.
Sadly the rate at which my eyesight is deteriorating would mean I would be effectively blind long before then.