No can do on 501. Sorry. It is capped at 6.3(5) as the latest it will run. 501 and 506 will never be able to run 7.x code. They are being killed off and replaced with the new, agressively priced, ASA5505
Looks like Remote Desktop/Terminal server is your best bet at this point if you don't want to buy the ASA
Depending on your license on the 501 you may also be capped at 4 simultaneous VPN tunnels.