Advice on my low gears for my Retro 531 build.

[widowmaker]

If you are running down tube levers then almost any old mech will work as long as it is long enough to cope with the chain length of a wide range of sprockets. Even if indexing or using STI you can mix and match road and MTB up to and including 9 speed. Older Shimano MTb mechs are just junk value. Buy a scrapper MTB and strip it.
Why not fit a MTB chainset?
Older front mechs are much more forgiving than new ones. My Campag Record one works quite happily with a 22/32/42 triple for example.

Thanks for the info !

I have a few mtb front and rear mechs in the spares bin, I wasn't aware you could mix and match so that's good news. I shall try the tourney rear mech I have, its new and cheap and cheerfull it can accommodate 34 teeth. So with a 59/36 front crank and a 14-34 I should cope with most hills, if not I have the option of fitting a mtb triple.