Lecture material of another professor
The question is: why wouldn't you send an e-mail? Consider you send him/her an email for permission to use the lectures, he/she may:


(most likely) Accept your request providing you reference him/her. He/she might even point some possible errors in the slides.
Deny your request. In that case you just avoided an awkard situation, would he/she ever find you plagiarized (if you modifications don't change the style). 


If he/she denies, should you still use his/her slides as a guideline (assuming you do the slides yourself)? Yes you can. And if you wish you can also reference him/her as a reference, in the same way you reference books.