Andrew,
I first bought the Roco BR57 at the Wigan model railway exhibition in 1990, and it quickly became one of my favourite locos due to its exemplory running qualities, especially at slow speed, never knew it to stall! A change of interest led to it being sold a few years ago, but last year Roco released the SNCF version (050B) which has the same mechanism as the BR57, indeed it is a BR57 but in SNCF livery. I quickly snapped one up, and again its running qualities are impeccable. Therefore I thoroughly recommend one to you, if you go second hand avoid the earlier ones (those with a ref no that ends in the letter A) merely because the older Roco models are just not up to comparison with their models from 1990 onwards.
Being a strictly DC analogue man, unable to help with your decoder query, but do get a BR57 you won't be disappointed.
regards
Clive