Expansion cards: In theory, no, you don't need to buy USB-C modules but it's highly recommended. At the back of each expansion card slot is a fully functional USB-C port that the card plugs into and if you need to, you can plug cables directly into them. But because those port are, as said, at the back of the expansion card slots, this means you'd have to turn your laptop upside down every time you want to plug something in. Also, accessories that are thicker than just a cable with a standard plug (such as USB-C flash drives, card readers, and so on) probably won't fit. Another downside is that having four expansion card sized gaps under your laptop is just ugly.

I would highly recommend that you pick up at least four expansion cards so all slots are filled. My standard setup is 2x USB-C, 1x USB-A and 1x HDMI. I have another 1x USB-C, 1x USB-A and 1x ethernet in my backpack in case I need to swap something.

Memory: The type of memory you need depends on the mainboard you choose. The intel ones need DDR4-3200 SO-DIMMs, the AMD ones need DDR5-5600 SO-DIMMs. Note the difference between DIMM (physically larger, for desktop PCs) and SO-DIMM (physically smaller, for laptops). Capacity-wise, they all have space for two sticks of memory with up to 32 GB each. If you're unsure, you can buy framework laptops with the RAM included. It's a bit more expensive than buying separately but you can be sure everything fits.