I'm not sure if the button I have in mind is even doable in Professional edition (or at all), but here goes:
I want a button on the Lead object that:
1. looks at the Lead Source field
2. Sends an email from an existing template (which template is based on the value of the Lead Source field)
3. Changes the value of the Lead Status field (same change regardless of Lead Source)
If a single button can't do all this at once, can it at least be set up to just send the email?