I’m not so sure I would recommend 7Digital anymore since they’ve been delisting a lot of stuff lately, meaning even if you already bought it, you can’t download it anymore.
Also, if you plan on building up a mainly digital collection, I wouldn’t use lossy codecs like MP3 for my main files, instead opting for FLAC or some other lossless codec, which can then be safely transcoded to a lossy copy at will.
You could always get binders to stick them in to save space that way.