At the engagement parties I've been to, presents have been expected. I wouldn't even think about not bringing a present. However, not everyone knows the rules of engagement parties and weddings, I certainly knew absolutely nothing about weddings until a couple of years ago. It sounds like you just really didn't like these people that much to begin with, they don't sound like they are that close to you, I don't think you should have to invite people to your wedding that are going to make you feel uncomfortable on the day. Certainly don't feel bad about not inviting them. If they question you about how they were not invited, you'll just have to tell them that you needed to keep the numbers down, that you had to cut corners somewhere, and that you're sorry (even if you're not). If they don't take it very well, well then it is pretty obvious that they don't care about you, and that you made the right decision. I have a very good old friend who I know won't be inviting me to her wedding, and I'll be doing the same, and I am absolutely certain it's not because she isn't a good friend, it's because she wants to save money. We will probably catch up again after our weddings, and admit it to each other!
In as far as what guests to invite, and who to include in your bridal party and all that, I've found that following your heart and being truthful about your decisions works out for the best
just don't over-explain your decisions to people (in person - over-explain all you like here!).