Why even a tablet?
Buy a pre-owned large screen Android phone that supports SD cards.
Samsung Galaxy Note 3 or 4 should do the job, plus they have user replaceable battery.

There are plenty of free audio players like amazing "FOLDER PLAYER" and others. Boot times are much faster than Windows tablets and overall much more stable if to use just as a playback device.

Windows tablets are trickier. Personally, I would go with Dell Venue 8 or HP Stream 7". Used, about $40-75. Or larger Dell Latitude 10 about a $100 used.

I have Venue 8 that I carry with me outdoors summers and winters. Never gave me any problems. Also, I picked up HP Stream 7" as a backup for $40. The nice feature about HP, that battery is replaceable.