Nice site, a little slow waiting on each drop down menu to load but its definitely much more impressive than any capacity calculator I've seen yet.
Perhaps you could have a custom section for the forklifts that aren't available in the drop down list.
If there was a disclaimer stating that it was just a estimate and for accurate results the user should contact the OEM. It should cover you.
Same for inputting the attachments. Limited amount of attachment manufacturers shown. Plus a lot of the time people get customized attachments made.