Yes, I would go for the OLED. There is no OLED with 64 GB, only with 512 GB and one TB. Alternatively, you could go for the 64 GB IPS model, which is quite cheap.

Every Steam Deck can be expanded with micro SD cards. I don’t notice a difference where the game is stored, but I also don’t track startup times with a stopwatch ;) But please buy a decent micro SD card if you plan to extend your storage.