Find out what you want in gearing...are you using the same rear wheel the bike came with? What is the range of freewheel teeth. Most older touring/sport bikes came with 14-28 freewheel, and a 52-42 (or 40) crank/chainrings. If you are keeping a double up front, any rear derailleur will do...even short cage. Add a triple and it becomes tricker.
As for Huret Challenger, they were decently well regarded as bullet-proof, but I have never seen one in action.
Some folks swear by Huret, other swear at them.
__________________
1987 Crest C'dale, 1987 Basso Gap, 1992 Rossin EL, 1990 Van Tuyl, 1985 Trek 670, 2003 Pinarello Surprise, 1990ish MBK Atlantique, 1987 Peugeot Isoard, 1987 Nishiki Tri-A, 1981 Faggin, 1996 C'dale M500, 1984 Mercian Pro, 1982 AD SuperLeicht, 1985 Massi ?, 1988 Daccordi Griffe , 1989 Fauxsin MTB, 1981 Ciocc Mockba, 1992 Bianchi Giro, 1977 Colnago Super, 1971 Raleigh Internat'l, 1998 Corratec U+D, 1991 Peugeot Slimestone, 1987 Bianchi Volpe, 1995 Trek 750