You have the option to buy most ships with real money, but the general cycle is about 6 months after release into the persistent universe the ships are purchasable with in-game money. The only reason to spend real money on SC is if you can’t wait those 6 months, want to support development, or don’t want to bother with in-game money for whatever reason. There are some exception ships though.

As for the detail, there are big differences between SC and ED. For one, SC ships have completely modeled interiors since the intended gameplay is for you to manually board your ship from outside. ED has no ship interiors as far as I know, just cockpits and exteriors, no matter how big the ship is. SC also has more ships than ED even excluding all the SC ship variants, ground vehicles, and ships that don’t do Quantum jumps, the frame shift equivalent of ED.