How far can I upgrade the RAM?
According to Dell, 32GB
No indication of wifi (802.11). Not built in?
Nope, NOT built in.
These are business systems intended for office use. Of course you can use them at home, too. Most business systems do not have WiFi built-in; you can always add either a USB WiFi adapter or a built-in card. As of the time these were shipped, most office environments insisted on wired Gigabit ethernet and did not encourage WiFi; though that has recently been changing.
Also it has DisplayPort as that is the typical port used by commercial monitors. This one is unusual because it also says it has HDMI – many older business Dells do not – they only have DisplayPort and VGA. Note that a $5 cable or adapter can convert a DisplayPort to HDMI so it’s not a big deal either way.