Couple of things:
Not sure why it wouldn't work on Windows 7 for you, are you selecting an x64 based payload if your using a x64 based OS? I have it running on mine without a hitch...
In the instance of your website, you shouldn't need to bother with ARP or SSLStrip, all SET does is bind to '' on all interfaces, you can force it to bind to a specific IP, try editing the config/set_config and turn AUTO_DETECT=ON to AUTO_DETECT=OFF and when SET loads, try configuring your interface manually, I've seen that be an issue sometimes..
SENDMAIL is only used when spoofing source addresses when sending emails, if your integrating into GMAIL, I've seen that STARTTLS error before, are you running the latest version of SET? If so, I must have no fixed it, some gmail smtp servers require you to use STARTTLS to enable secure transport, others don't and won't accept it, its been a weird quirk to work with on gmail.. Let me know and I'll try to fix.
In the instance of sending to GMAIL, a sendmail attack won't work, make sure the config/set_config SENDMAIL=OFF is turned on, GMAIL does reverse lookups, so spoofing your source email address won't work properly.
Hope that helps.