To answer the question "Which PI?"
Assuming you are speaking of a Packet TNC 1200 baud.. any Raspberry pi has plenty of Horse power. A RPI4 is actually overkill...
I use and have used for at least 8-10 years a RPI2 with a Coastal Chipsworks TNC-PI kit, that I have had no problems using for APRS and Packet. It just keeps playing...
You can find RPI2's and 3's for less than $35 retail price. Ebay is a source.? I bought a few RPI3B+ for $27 at Microcenter about January.
I also recommend VILROS.COM for a mail order source. I have used them 4-5 times. They are in NJ and sometimes have free shipping options. NJ to KS where I am located has been 2 says all 4-5 times.. So people on the east coast can probably get 1 day service. Standard shipping is USPS.
Larry W8LM
Larry Macionski