A friend of mine has recently been in the same situation,he needed a very simple phone for limited purposes.
He got himself a new Oppo A57 from Lazada, about 1500Baht.
The phone does everything that is required, all Apps are working, 2 SIM slots AND a memory card slot.
Mind you, that model is already a few years old, so you won't get any OS updates anymore, but works fine, fast GPS, good battery life and fast response.
For something a bit modern and still budget-orientated, check Lazada (or any other store) for a Xiaomi Redmi 9.
Its various models start around 3000Baht.
I very much second the recommendation of not buying a second hand phone, prices are too high, quality is dubious.