No, as far as I know, it's not possible to add another NIC to that machine. If you are pushing 100GBytes/day through that one NIC, bear in mind that its 1Gb/s bandwidth will be pretty much split between WAN and LAN. If you aren't on gigabit fibre, it probably won't make much difference.
You can use VLANs on ESXi in much the same way as you do with your managed switch.
You will probably want to allocate at least two cores to pfSense, given the packages you run.
I don't know much about the disk space or memory requirements for those packages but I suspect 6GB of RAM would be plenty. Someone else might be able to help with those numbers.