Hi,
Isolating or not?
If non isolating, then LM2596 is a good choice (besides thousands others).
DCDC is O.K. you just need a rectifier and a capacitor to get DC as input for the LM2596.
Adjaustable is O.K. too. It juast says that you can set up output voltage as you like (in a limited range)
Mind that with 48V RMS sinewave input you get about 70V at the capacitor after the rectifier.
LM2596 can stand 45V max only. Thus it´s sutable for max 30V AC RMS sinewave only.
Why do you want to design your own? There are ready to buy modules available from very cheap to very advanced.
With or without case, isolated and not isolated.
ICs are available. Thousands. Thus the distributors and IC manufacturers provide interactive selection guides for you. .. and for free. Use them.
Just type in your requirements. Don´t be afraid when it shows 100 of suitable ICs. Be happy instead ... and choose the one that suits you best.
Klaus