Also got a floppy drive from my friend (External 3.5" Apple).
Please make absolutely sure that drive is
compatible with a IIGS before you plug it in.
Any Mac serial MIDI interface should work. There are even DIY designs around if you don't mind building one.
Or guitar jack-ish controller?
Do you mean an audio input, for recording "live" instruments? You should be able to make or buy a simple plug adapter to go into the IIGS's audio input (mic socket?) - then it is just a case of finding the right software to record audio and edit it.
Not directly music related, but for storage, bigmessofwires here on the forum makes an SD card storage device which works with the IIGS, emulating a floppy drive or a hard drive. You can find more information about it, and buy it,
from his site here.
Another option is to use a cheap Linux computer (like the Raspberry Pi) and run A2Server on it - you can boot from it, and also use it to connect to the internet, download files and store them on the R-Pi's SD card, or if you are using a different computer, on it's hard drive.
you will need 2 3.5in drives and at least 1 5.25in drive.
Why?
it is compatible to the all the older Apple II software from before.
This is true, but none of the older Apple software will access the onboard Ensoniq sound system.