Wow, a brand new user! Welcome!
Trashcan is really not necessary. It's just an optional directory where you can put things before deleting them (with the Empty Trashcan menu option). I've never used it. Unlike Mac/Windows, when you delete something on AmigaOS, it's deleted - it doesn't go to the Trashcan as an intermediate point unless you put it there directly.
(SFS and possibly PFS - other optional Amiga filesystems - do have a more Mac/Windows-like recycling option if that's something you're interested in)
My OS4 install does have a swap partition, but as far as I can tell it's never been used by the system. I think it's safe to go without, but if you do want one I'd suggest one either equivalent to or double the amount of physical RAM in the machine.
EDIT: I should also add, if you did select the option to create a Trashcan and decide that you don't want it you can actually delete the Trashcan, although I think it has to be done from a Shell or a file manager like Directory Opus. Likewise, if you didn't select the option for a Trashcan and do want one, you can create one without having to reformat. Just create a directory called Trashcan, and either change the icon type to Trashcan from the icon information window, or replace its icon with a copy of ENVARC:Sys/def_trashcan.info
