No, you don't. But you will need a managed switch. In order to use 1 ethernet with 2 interfaces (LAN+WAN) you must configure 2 VLANs.
If you plan to use additional VLANs, the need of a managed switch is independent of using an USB-ethernet adapter.
Otherwise make your decision depending on your budget: an USB3-ethernet is about 15 €, a managed switch about 20-25 € (5-port) or 25-35€ (8-port).
The maximum speed of all VLANs is added, the limit is the maximum of your ethernet speed. Example: if your modem (on WAN) delivers a max of 100 Mbit/s, then you'll have a range of 800-900 Mbit/s for your LAN.
Of course. In that case your speed will be limited.
There are USB-100Mbit-ethernet adapter (less than 10 €), those are mostly USB2 (max speed: 100 Mbit/s).
There are USB-1Gbit-ethernet adapter, those are mostly USB3 (max speed: 1 Gbit/s).
You can connect an USB3-1Gbit-ethernet adapter with an USB2 port. In that case you'll get a maximum speed of about 300 Mbit/s (with a power consumption of about 0.3 W, connected to an USB3 port will use about 1.0 W)