Rebuild 5.7 or buy something else?

if you have never done it before, i'd take all the parts to a machine shop. let them evaluate and give you an estimate.

you can have them prep the pieces and then put it together yourself.

any way you look at it, it's going to cost money. with guidance/experience of the shop you are spending your money wisely. there is a point where you can try to scrimp and save too much and do it all yourself and end up losing.

i put my first 340 together, back in the 80's, i had a machine shop hot tank the block. bore it over. balance the crank/rods/pistons for me. i had a book that told how to check clearances, use plastigauge, etc. how much to torque everything, yadi yadi yadi. it's a lot of attention to detail. everything cleaned, everything clearanced, torqued etc just right. different lubes on stuff. anti-sieze on certain bolts, oil on certain parts. prelubing with a drill and extension through the distributor hole. piston ring gap.

lots of stuff to look out for... you need excellent documentation and patience. cleanliness is key. no metal shavings, etc. good gaskets.

the benefit of using a good machine shop is they do this stuff day in/day out it's like second nature to them.