I'd start by getting a reloading manual. That'll lay out what needs to be done and will help you figure out what is essential and what isn't.
At a minimum, for bottle necked rifle ctgs, you'll need:
a manual
a press (single stage would be a good idea--ancient and used will work fine)
a set of dies (I'd go with a FL sizing die)
a shell holder (257 is the same as 30-06/308)
a powder scale
some way of dispensing powder, be it a powder measure or even scoops will work at first
case lube
a way of seating primers (built-in function on some presses)
6" caliper
a case trimmer
I'd buy new dies and caliper. Scale, powdermeasure and press I'd buy used if you're on a budget. eBay, craigslist, etc.
257 Roberts factory ammo isn't super hard to find; big chain stores might not have it, but I've seen it around in most dedicated gun stores. 6mm Rem brass is around. And sometimes 7x57mm. Both of those you can run through the 257 Roberts sizing die and make what you need.