Building a voice amplification system

The situation: I have a disability that makes it very difficult for me to talk loudly. I've started avoiding even moderately noisy restaurants or public events because it's almost impossible for anyone to hear what I have to say, even if I'm standing right next to them. It's frustrating.

The idea: I'd like to build a simple voice amplification system that makes my voice louder. I could carry it with me to noisy places and turn the volume up to make it easy for other people to hear me. Some systems like this already exist, but all of the good ones seem to be over $500, which I'm a bit hesitant to pay.

The questions:

1) What kind of hardware would I need to build something like this? It seems like a battery-powered speaker with a microphone jack might do the job, but I've been unable to find anything like that.

2) Does it need to be a unidirectional speaker? It might be kind of weird if I was sitting at a restaurant, and the people sitting behind me could hear me just as well as the people at my table.

3) Would there be a significant delay between me speaking and the sound coming out of the speaker? I suppose you would get used to it, but I'd like to avoid it if possible.

Thanks for your help!