Roland mentions he saw the Ecto-1 at an auto show once. Ecto-1 did participate in an auto show in Follow That Hearse (1988).
Achira received an entry in the 2016 tie-in book "Tobin's Spirit Guide - Official Ghostbusters Edition" on page 75, in Section IV: Gods and Mayor Demons. It is identified as a Class VII Noncorporeal Possessor.
The bum on the park bench who is rejected by Achira will be seen again in The Unseen (1997), when he is questioned by Kylie and Eduardo on the same park bench.