1. You can use Sidecar wirelessly, or with a cable connecting the iPad to the Mac. A 2018 MacBook Pro has USB-C ports, so one end of the cable should be USB-C. The other end of the cable should match the iPad you have (for example iPad Pros have USB-C, older iPads have Lightning). A wired connection tends to be more reliable.
2. There may be a slight delay. The delay may be more noticeable if you connect wirelessly, because then it depends on your local network. Also, for some reason Sidecar sometimes randomly disconnects on me, especially wirelessly.
3. If you already have an iPad, then Sidecar is free. If you don’t already have an iPad, many iPads cost more than a small HD display, so then you should decide what the external display is for. If it is for monitoring while editing a video sequence, a small HD display might be better because it is 16:9 and optimized for common video standards. And as a true external display connected via HDMI (you’ll need a USB-C to HDMI adapter), there should be no lag. If you already have an iPad and you mostly want it for displaying user interface panels or other applications, then Sidecar can be good for that.
For more details about technical requirements and how to use Sidecar, read the Apple help document about Sidecar: