Sounds like you ought to join the Hornby Railway Collector's Association (HRCA). The association is dedicated to the metal Meccano built trains introduced by Frank Hornby first made at Binns Road Liverpool and later in factories round the world, including Argentina and France! The French really took to Hornby and were the last factory to make them. The Original Liverpool Hornby company folded in 1965 and the name was taken over by Triang in 1966, based in Margate Kent. This moderrn company is only 'Hornby' by name, the metal train tradition being lost and replaced by the plastic of Triang / Rovex.
The HRCA http://www.hrca.net/ is an enthusiast body to support those of us who still love and use the Meccano Hornby trains of yesteryear. They support and welcome international enthusiasts. There is a full colour magazine that comes out 10 times a year and they run auctions of pre owned Hornby products. Probably the most useful thing for you would be the Spares and Services directory. This gives the names and addresses of people many of whom are HRCA members who make and sell spares to keep your old Hornby trains in working condition, and sometimes offer services to repair them.
A few answers to your questions -
20V is AC, but will work on DC, they can draw a lot of current so you need a substantial power source. I use a bench power supply I bought from Maplins (an electronic chain store sadly recently shut down) .
I haven't looked at the French built locos so I cannot answer the question about the switches, my locos have two knobs which stick out of thr driver's cab. On clockwork one is brake the other is reverse, on electric only one knob and it is reverse only. And also for the clockwork they have trip switches underneath the motor which operates from trips in the centre of the rails. Some electrics can be reversed from the controller by pressing a button.
Beware of old transformers they can develop faults over a period of time. Get them tested before use.
24 V bulbs should be OK.