A full CD album of audio would be 650-700MB, so if downloading CD quality WAV you’d be able to store I suspect around 3,000 album downloads. Artwork and/or PDF booklets might add a few MB to each if they are available.
Long ago I used extra/internal and external full size drives, but for many years now I’ve tended to go for USB 2.0/3.0 portable bus powered drives - I have mostly WD ones (a few dozen probably) and a few Seagates of varying sizes.
For a few years I think the 4TB portable drives have given best bang for the buck - very acceptable capacity, speed and price/GB.
For under a £100 pounds online or say at Currys/PCWorld my favourites are usually the WD 4TB Elements portable bus powered drives - I have several of these as well as the Passport drives, but in all honesty I think the cheaper element drives are just as good.
Some small drives will have an encryption feature which has pros and cons - the Elements don’t normally - you are unlikely to want to encrypt audio files themselves, especially if you might hook up to a streamer in the future which will not be able to enter a password to access a drive.
The main issue with hardware encryption is that if the interface doing this fails you may not be able to salvage a good drive by sticking it in a standard USB enclosure, though I believe many 2.5" drives these days do not have a standard SATA interface anyway and may be more directly coupled to the enclosure circuitry.
Don’t think you can really go wrong with the majority of portable USB drives out there, though any of them could fail so make sure you have more than one drive with copies unless you can redownload.