Problem with getting a HD floppy drive is that the Plus doesn't have the required circuitry to run a HD SuperDrive as a SuperDrive - it has an IWM chip instead of a SWIM chip, and therefore a SuperDrive will only work as an 800k drive. There were some third-party HD floppy drives made that work with the Plus, mostly those by Applied Engineering, but good luck finding one of those. You're better off getting a zip drive for data transfer.

For software, I seriously can't believe nobody's mentioned ClarisWorks yet! I recommend ClarisWorks 2, its a lightweight app that will run rather nicely on a Plus, even in System 6, and does pretty much everything you need - it has a word processor, drawing module, paint module, database module, spreadsheet module and communications module (terminal program), teriffic little app