Upload & Sell: Off
You are right that the 350D ("Rebel XT") will make a fine starter camera for someone. For example, someone's kid might find it a great way to try out DSLR photography. For most people, the single 50mm prime won't be as useful as the kit zoom, but they can always pick up the other lens.
You have a bunch of options here if you are certain that you want a 50mm prime for your 5D2. Before you part with it, why don't you shoot that fine but humble little 50mm f/1.8 lens on the 5D2 and see what the results actually look like? It might be that you are happy with what you see - and, after all, the actual performance of the lens on your camera is more important that what people might say about the lens.
The 50mm f/1.4 is a fine performer on the 5D2. While I shoot zooms more than primes, I rely on primes for certain subjects and types of shooting and I use this lens a lot. Its image quality is overall excellent. All lenses have shortcomings, and this lens is not exception. It has excessive "halation" (a sort of a glowing effect) at f/1.4 that some mistake for softness - the issue disappears if you stop down even by a partial stop and the lens is certainly quite usable wide open. The lens shows a small amount of barrel distortion - you will rarely notice it, and when you do it is simple to correct in post. Neither of these are significant issues, and I enjoy and recommend the lens.
I have not direct experience with the 50mm f/2.5 macro, though I've heard a lot of good things about it. You'll have to decide how important that extra stop and a half is to you. If you mostly shoot small aperture stuff like landscape, you might not miss it at all. If you shoot macro and like the 50mm focal length for such shooting, that is a factor, too. (Most would probably want a longer focal length for macro work on full frame.)
Sounds like you'll have a nice set of primes either way. I also use and like the 35mm f/2 - a great little lens, and most of the oddball stuff you hear about it comes from people who are repeating what they heard others say rather than reporting their own experience. Rather than the 100mm f/2 I have a couple of primes with focal lengths on either side of that, but I've also heard a lot of good reports on the lens.
So for the longest time I had a 350D and used a 50/1.8 on it and loved it so much. (The 18-55 kit lens packed up on me and I was forced to buy the 50. It was obviously worth it.) Since then I've got more into photography (predominantly events and portraiture), and bought a 35/2 - which was just fantastic on APS-C and wonderfully sharp, though loud - and now a 5D mark 3 body. The 50's great, the 35's a little wide, but I appreciate having more of a wide angle at times. For portraiture, I wanted something a little closer so I tried a 100/2 and was hooked (it's just a dream to use on full frame and the price (£210 slightly used) was excellent) so I bought it.
Now I want to sell my 350D, mainly because I want it to keep being used and want someone to realise what a great piece of kit it is to learn photography on. I'd probably sell it with the nifty fifty, meaning I lose a pretty useful focal length. I was thinking of using the spoils from selling it to get a 50/1.4, but I'm not 100% sure. What other (if any) lenses should I consider and why? I've also seen the 50/2.5 macro for a similar price and while I hear good things about it's sharpness, I'm not sure I'd like to part with an f-stop or two.