You need to better define your project. It may be that a Raspberry Pi (which includes WiFi) is the ideal platform for 'management' functions while some kind of uC could handle the mechanics.
In any event, before considering the controller, how many inputs, what kind of inputs, how many outputs, what kind of outputs? How are you going to get along without the HMI (VERY useful peripheral). There are touchscreens that will talk to uCs and the Pi. I might keep the PLC for the mechanical functions it already controls. It would be neat if the PLC could communicate over serial to the Pi - most have modules for this but they tend to be pricey.
General purpose IO is not the Pi's strong suit. It has pins but either they perform some trivial function or you have to write device drivers and only the tip top professionals want to venture into that. And you can't get real-time performance from Linux so you really don't want it involved with the control loops.
The Pi will talk to HDMI but I haven't investigated Touch Screens very much. They are available:
https://www.adafruit.com/product/2718Again, I don't know how many GPIOs you need but there are many uCs with lots of pins. If there aren't enough pins, add another uC. Let the Pi be the boss and communicate to the peripheral uCs using UART, I2C or SPI which is already available on the Pi and trivial on the uCs. I would look into the PSoC chips - they have a lot of IO and very advanced features. The development environment is fun to use and there are a LOT of videos. Look into
www.cypress.com/psoc101That new Pi 4 with 4GB of RAM is a really fast machine (and it needs fan cooling, it's available) so it should easily be able to handle databases and management for the cart. Along with WiFi, of course! Very handy...
In short, partition your project.
ETA: Google for 'usb rfid reader raspberry pi' - it's been done.