Hi Aaron,
The biggest problem with the older systems is neglect - particularly in damper climates, if the car is left without changing the fluid for more than a couple of years, then you can be up for some major repairs, because of the corrosion caused by the moisture absorbed by the fluid. (And the general nastiness of the stuff - takes off paint etc.)
Similar neglect of an LHM car will usually just result in you having to clean out the filters and flush the fluid.
The other problem is that 'proper' LHS is getting hard to find. It is actually something like a mixture of old-fashioned brake fluid and castor oil. People like 'Buttercup' Bob in Tamworth make up their own substitute, and he has even experimented with Canola oil, with mixed results.
Yes - you can convert a car from LHS to LHM. I have just finished helping Aussiefrog 'DS' with doing just this to his 1963 ID19. It requires a lot of work and a reasonable knowledge of Cit hydraulics. Every moving seal and most of the others have to be changed. This was an ID with no power steering - if it was a Hydraulique DS it would be a very skilled job indeed.
I would think that if you were paying someone it would be an expensive proposition too!
The earlier cars are much more softly sprung though - less damping, resulting a wonderfully soft rolly car, that still feels OK from the inside. (Scares the S&*T out of people watching it go round corners fast though!)
head_ban
Paul