To help with the disc brake and power steering questions. Both are relative and subjective with many options. That said, if you plan on keeping the straight axel this is what I used with good success: I just installed a Classic Performance Parts (CPP) vacuum assited power disc brake kit on my 1956. The install was straight foward, all the parts fit, and the difference in stopping power is huge. This install does requires cutting the brake lines. Fittings, tubing bender and a flare tool are required and not supplied in the kit. I replaced the hard lines with stainless and got a complete kit with fittings, adapters and tubing from Inline Tubing. CPP probably has this stuff also. They failed to mention these were needed when I was buying the brake kit so I got it from Inline Tubing. After much research I did get the best deal for the brake kit and booster from CPP though (There are lots of vendors with this kit). I too have a 3 speed transmission and that did give me a little difficulty. I believe the 57 and 56 have the same clutch linkage set up with the brake riding on the same shaft. If you plan on keeping the manual trans and pedals you will need the kit that allows this. The difficulty came with removing the old shaft. A long story short the tapered, slotted retaining bolt could not be removed while in the truck so I had to cut the old shaft out in order to remove this special bolt to use with the new shaft.
Sorry about the long winded response...I guess to answer the question I should have just said CPP's kit is good.
Again assuming the straight axel is retained and the 3 speed is column shift, then the power assist ram type power steering is the best bet. It is not cheap though. There is a kit using a Toyota box that is supposed to be good but requires cutting the column and with "3 on the tree" would cause problems. I'm not sure it would work on the 57 though.
Anyway, I hope that helps.