Aha!

All those replies got me thinking and reading some more.
I understand that you need a certain working distance from your subject in order to eliminate distortions (huge nose/small ears etc.) which probably explains why people commonly seem to use 85 and 135mm primes instead of a Canon 35mm f/2 which I intended mainly for indoor (low light) portraits. No wonder I've been I've been giving myself a hard time focusing on fast moving kids with such a close working distance. And I've probably also distorted their faces in the process as well. But.... I have gotten to like the idea of using primes instead of just zooms all the time.
So, rather than having to guess (or remember the distance from past experience) at which distance from the subject a lens distorts I'm thinking that say an 85mm lens
forces you to keep a specific distance at which the subject isn't distorted while with a zoom (say my Sigma 17-70) you can always zoom out, move closer and probably mess things up. A 70-200 would be different of course as I just tried it with my Canon 70-200 f/4L. With my 1.6x crop sensor I found that even at the minimum 70mm (112mm equivalent on a FF) I got very close and was forced to back away in order to frame the subject, thus eliminating distortions.
Zooming in closer to 200mm was pretty hard as it made for very tight headshots and was also hard to hand-hold (I don't have the IS version). At 85 mm (trying to emulate the framing of an 85mm lens) it was doable, but of course not easy peasy as with the 35mm where hand-shake isn't an issue and I can just snap away (it is of course a lot lighter than the 70-200 which helps). 135mm was even harder and needing more space (OK in a studio I guess but harder in a small home). But then again I'm guessing most serious portrait photographers go FF.
An 85mm would be a better choice for a 1.6x crop sensor, wouldn't it? And still usable for a FF (the equivalent of how 53mm looks on my crop camera if my calculations are correct). This looks good to be suitable for both a headshot and a full portrait. The 85 f/1.2L seems like an unnecessary investment at this stage. I can always reconsider it when I get more experience, so the much lower priced 85 f/1.8 comes into consideration. According to Ken Rockwell, it's very sharp, and
much sharper than the 100 f/2 (comparison at f/2) which brings me to yet another lens; the 100 f/2.8 macro which is, according to many reviews/comments, also a nice portrait lens (even though it obviously doesn't give the same shallow DOF as the 85 f/1.

. But it would cover my needs for a close-up lens (I'm thinking food photography, rings etc. in a wedding etc.) The 100 f/2.8L IS is tempting, but having read that you really need a tripod and a flash in any case when doing serious macro work, and it seems a bit over-priced for a plastic shelled lens I think I'll pass and go for the non-L/non-IS version instead.
Next I'm looking at a general-use lens which should replace my Sigma 17-70 which is pretty worn and not a lens I'm very happy with any longer. The Canon 24-70 f/2.8 seemed like a good replacement but after some reading doesn't seem to be "the perfect allround lens" any longer with sharpness and build issues according to many user comments (then again lots of people seem very happy with it) which is why I've been eagerly awaiting the 24-70 f/2.8 II. Not any longer with that price tag. I'm thinking that with an 85 f/1.8 I'll have a sharp and shallow DOF lens for portraits. Along with my 35 f/2 I will also have two "indoor primes", so the 24-105 f/4L looks like a suitable replacement for everyday use (outdoor or flash indoor "do it all" lens). And unlike the Sigma 17-70 it'll also work on a FF. On a FF it should give the same view as 15-65mm on my 1.6x crop camera; a little wider than what I already have but not quite as much reach (and a lot less than 105mm on my crop camera). But no worries as that's where the 70-200 f/4L comes in.
So that leaves wide angle where I see one option for now: the Canon 10-22 f/3.5-4.5. On my 1.6x crop camera it should give the FF view of 6-13mm which I can only imagine is amazingly wide and should be great for stuff like real estate and also for "getting in with the action" type shots if I additionally get really close to the subject(s). It seems that the Canon 8-15 f/4L would be the closest to get for FF use. Funny that with this setup I would be getting 2mm wider with a crop-only type lens

This is getting veeeeery long, so to conclude what I'm thinking would work with my 50D and also a FF in the future:
- Canon 85 f/1.8 (portraits)
- Canon 100 f/2.8 (macro/close-up and portraits)
- Canon 24-105 f/4L (travel, general use)
- Canon EF-S 10-22 f/3.5-4.5 (real estate, indoor, "getting in with the action" wide angle)
- Canon 70-200 f/4L ("getting closer" lens ("getting very close" on crop) in studio, outdoor or with flash -I already have this
- Canon 35 f/2 (indoor (low light) wide angle (FF) or low-light general purpose (1.6x crop) -I already have this
Then, when I get a FF I'd sell the Sigma 17-70 and 10-22 (or keep one of both if I'd still be keeping the 50D), then get an ultra-wide such as the 8-15 f/4L if I'd need that for my FF.
For lighting, here's my proposal:
- Canon 580 EX II (on-location/portable flash setup)
- Canon 430 EX (on-location/portable flash setup -I already have this
- Phottix Strato II manual radio trigger (with TTL pass-through) on-location/portable flash setup
- Stands, umbrellas, brackets for the above -I already have some of this (umbrellas and stands borrowed from the Elinchrom gear below)
- Elinchrom 500 w/s BXRI heads (2x), stands, 2x small soft boxes, 2x umbrellas, reflectors, snoot, barn-door, Skyport radio trigger (for studio shots) -I already have this
- Manfrotto Auto-pole 2 (for studio backdrop mounting, allows flexible repositioning and mounting of other devices as well if needed) -I already have this