not into the rando-thing, but i did train through two wet scottish winters.

sounds like you're planning on wearing too little on top.

if it's in the low 50's then i'd wear:

base layer
ss jersey (this is might be too much, but better safe than sorry)
ls jersey
outer shell (or maybe just a vest)
tights, or if you feel really hardcore knickers (keep the knees covered).
booties.
long fingered gloves.
cycling cap, under the helmet
glasses

if you have some kind of embrocation, then that might be an idea. it may make the tights a bit nasty, but you're legs will have an extra layer of protection.

if you can cope with it psychologically, then it shouldn't be too bad. 50's is warm, at least to me right now