If you go for a disk system with a PEB, it's probably worth reading up on the various options available. There were (basically) three disk controller cards available.
TI - 90-180K, sings density, double-sided (original TI disk drives are single-sided only)
CorComp & Myarc are double-density, but each uses a different format. The Myarc card was a little faster.
If you find packaged software, the bulk of it is likely to be SS/SD 90K format, the lowest-common denominator.
I've grossly over-simplified...others might chime in. I recommend some type of disk system or disk emulation, though, because there was a lot of great software available on disk that never made it to cartridge, especially software released after Black Friday. I'm thinking software from Asgard, Comprodine. The TI Game Shelf website has a pretty good collection if you want to see what kinds of things are available.