A couple of years before I was born my aunt gave birth to a girl whom she placed for adoption, and recently I was finally able to meet her. I know my aunt is beyond thrilled to reunite with the daughter she said goodbye to so many years ago. Personally, I am so glad I got to meet my cousin. I loved getting to know her and her husband. I loved sharing stories about our grandparents. I loved listening to talk about her life and her family. I loved it!
If you want to know more about your dad and his family, go for it. I suspect there's no way to guard your heart on this one. No matter how hard you try, if they are not interested, it's going to hurt. But I don't believe you're going to feel any better if you never try. I think, instead, you would be living with regret.
I understand that you shouldn't "expect to be welcomed after 30 years," but that doesn't mean you won't be. The thing is, you don't know what they know. Your mom said that they could have contacted you and chose not to. I'm not so sure you know that for certain. You don't know what your dad said. You don't know what he thought you or your mom wanted. You just don't know.
My cousin was welcomed (wholeheartedly) after 40+ years. Why wouldn't you be welcomed, too!?!
You won't know unless you try ...