the netgate should do it. Not sure, if the Network cards support VLAN but you probably do not need VLANs.
One NIC is for WAN, one for your home network and the last one for your Wireless network.
The CPU power ist not that much so it would be interesting how much bandwidth you have on WAN and if it is important for you to route high traffic between LAN and guest network ?
Configuring WAN and LAN should be no problem and I do not explain that.
The guest network - I would do it like this:
Connect a LAN port of the WRT54 to the guest-LAN port of pfsense. So the WRT54 is just like a switch/bridge and is not doing routing or something. So your guests will get the IP from pfsense DHCP server and the firewall rules on pfsense will work for them.
Doing VPN (OpenVPN or IPsec) is possible with pfsense. Throughput depends on your needs and on your internet bandwidth.
But with this less CPU power you will be probably not able to use much more and CPU intensive packages like squid or something else.
Hope I could help you and welcom to the forum