Questions about Apple Mac mini

Yes, the working memory can be increased later.

Hello the Mojave system is very old (10.14) in the meantime came out Catalina (10.15) and since October Big Sur (11.1) this computer is compatible with these updates that you can do for free but I advise you this computer instead which has the same characteristics but uses the new M1 chip from Apple which stops using Intel processors and it costs 800. Apple Mac Mini - 2020 (M1, 8GB, 256GB, SSD)

Yes

Any display will do... There are USB-C to HDMI or DVI cables :)