JMH,
Actually, the 124 suspension is 99% different. About the only thing the same is they both use lower ball joints and have a sway bar. Really, they are THAT different! The 124 suspension is simple and easy to work on (IMO) while the 123 is a real pain. I did a full 123 front end rebuild last year and it made me appreciate the 124's simplicity.
Anyway, guide rod mounts are about $60 each plus a couple hours labor. How in the world does that turn into nearly $600, even with an alignment? Wowsers. You can do the job yourself, it's not easy, but possible. Mine were TOTALLY shot - I bet yours are too, they don't normally last beyond 200kmi!
Oh yeah - 124 suspension noises can be sway bar bushings or ball joints, but that's usually in a straight line over bumps or while braking. Turning the wheel, I dunno, maybe idler arm? Need to have someone help listen while you turn the wheel (or vice-versa).