Glad to hear you're starting into the wonderful world of growing. Please remember one thing in your venture...Your security is the most important thing you have, so protect it. With that being said...here are the answer to your questions:

1. No, you don't need multi-spectrum for flowering. Ideally, you want Metal Halide for the veg. stage and HPS or High Pressure Sodium for Flowering.
2. The difference in the two is the spectrum...MH puts out a more blue spectrum which the plants need for optimal vegetative growth and HPS puts out a more red spectrum which is most optimal for the flowering stage.
3. Answered in 1 + 2.

Hope that helped...let me know if you have anymore questions...
-sD