You would also need to make sure you DHCP config has a routers option to tell your DHCP clients (LAN users) their default route is the IP on your p2p1 interface. As well, the box itself will need a default route to the what ever is assigned by your PPPoE.
I generally prefer to make my iptables much more restrictive than that. (And the -P all fails on my CentOS 6.4) At a minimum, I would recomend changing the last iptables statement to something like the following:
To limit NAT to traffic recieved from a given interface.
iptables -t nat -I POSTROUTING -i p2p1 -j MASQUERADE
To Limit NAT to traffic from a particular subnet
iptables -t nat -I POSTROUTING -s 192.168.0.0/24 -j MASQUERADE.
substitute the 192.168.0.0/24 with whatever address space your LAN users are using.
You probably would not need the "iptables -I INPUT" & "iptalbes -I OUTPUT" lines.
You may also need to edit /etc/sysctl.conf
Look for a line with :
net.ipv4.ip_forward = 0
change the 0 to a 1
net.ipv4.ip_forward = 1
If you don't find it, add it.
sysctl - p /etc/sysctl.conf