From what Ive read GargravarrD, Retropie is one front end and Motion Blue is another. I watched a 'tube vid on it from DrewTalks. He preferred Motion Blue but he went through the pros and cons of each in list form so it was a good video to learn from.
As for which ROMs, some packs have this others have that. Many have the same but may focus on different consoles. Some just have ROMs for the one console.
Get a big pack like 128Gb and you get a lot of ROMs for a lot of consoles, but still not ALL ROMs for ALL consoles.
Whatever sized game pack you download you might want to get more of a specific console. Th big packs usually have all available ROMs for particular/popular consoles as originally after a console was released there was only a certain amount ever sold until superceded or obsolete.
Having Nintendo 64 ROMS in a Raspberry Pie pack is a bit sill imho as the Pie doesn't do N64 very well at all, ask anybody. BUT the emulator could get improved or a better Pie (2019 I heard) then your N64 ROMS will prolly work or work better but why download them now unless you will be using them in a more powerful system like say . . . . an Nvidia Shield ?