I'm gonna throw in a "ditto" and say that I pretty much agree with what's being said here. I think Joyce was wary of Angel because she never saw his good side. Instead, she saw his totally-unhealthy-for-my-teenage-daughter side. Of course that raised her hackles a little bit. Joyce's internal alarm system started going off when Spike became interested in Buffy, too. Who can blame the lady? My mom wouldn't pick an undead man for me, either.
As far as Spike and Joyce go, well, they had a neat relationship. I think Spike needed a mom. He was a mama's boy in life, after all, and even after he was turned, he wanted to turn his mom so she could tag along on the Spike and Dru inaugural killing spree of 1880. Killing his mom (twice) had a profound effect on Spike. I think it aided in the transition from sensitive William to Big Bad Spike. In Joyce, I think he saw a mother who was coping with the supernatural. I always saw Spike as sort of the Buffyverse answer to Little Women's Teddy. He badly wanted the female companionship and the sense of family that both Joyce and Dawn provided. He loved them because they were normal and decent and caring people in an abnormal, indecent, and cruel world. They treated him like a man, so he treated them like family.
For Joyce's part, well, I think Joyce needed a friend -- a friend who was also on the periphery of Buffy's life. Joyce and Spike both knew what it was like to be on the outside looking in. They also knew what divorce was like. What else are you going to call it when you split from your partner of 100 years? Spike always had a very decent, classy, human side to him. Because of how he felt about Joyce, that came out around her very quickly (far more quickly than it could around a slayer). Joyce was able to bond with William, I think. William was the sentimental man who came around to help Buffy save the world just cuz he liked the little things in it, the man who came around to talk about love with Joyce over hot chocolate and marshmallows, and any other little interactions that he had with her. I think part of her saw that William needed a mom, too, and that must have been a welcome feeling, with Buffy constantly pulling away.