all 510s are RWD
your other question is hard to answer, it all depends on what is available for sale in your area.
your profile says that you are located in so.cal, if that is true you shouldn't have too much trouble finding a nice runner or a shell. your main focus when checking out these potential project cars is the body/steel, if it is good, everything else is replaceable. look for dents, excessive bondo, and rust (especially in the floorboards, wheel wells, and spare tire well in the trunk) other than that little bit of advice, i would need specific questions to answer.
welcome to AF and the 510 forum.
good luck on your search.
if you have any other 510/datsun questions, feel free to ask, i'm allways happy to talk about 510s.
oh, BTW, how much is the guy selling his L-18 for? how recent is the rebuild? how many miles?
(not for me, just curious)
i'm running a fully built and balanced L-20b with dual sidedrafts and i just installed a subaru LSD last week (sweet)
.....a picture of my carbs ..... oooh, shiny.