Firewall Recommendation

Hello all,
Looking at getting a new firewall for our 150 person company. We currently are behind an Intel Lan Rover Gateway Plus. However, that system has been discontinued, to some extent by Intel. We really need a system that does NAT and will allow VPN Clients through the NAT to our customers Cisco PIX/Nortel firewalls. We also host some low use Websites (internal) which have firewall rules to allow access only from certain IP’s. Anyway, have looked at Cisco PIX 515, Watchguard Firebox 1000AND MS ISA server. Looking for something that would be 6K or less.

Thanks,
Matt