If you have an iPad gen 1-4 you can use use a kit like this https://www.aliexpress.com/item/4000539812281.html to use its screen as a monitor. (that kit is only for gen 2 so find a different one for a different gen)
Depends what you want to make. Some popular designs are the G-boy (left) and the Ashida (right).
G-boys are pretty expensive nowadays though, because they've been discontinued.
You could also make something like this if you want something simple and cheap.
Those links R. G. Marigold posted...