This is just my personal opinion.
I recommend everyone get 3.5 power loupes to start with. I started with 2.5, but I've been working with 3.5 for 6 years and I wonder why I ever did the 2.5. I have great vision, so I don't need the extra power for vision purposes. I just find the 3.5 to be more useful. I also have a set of expanded field and regular field. I think that just depends on your personal preference. Most expanded filed loupes will be heavier. I prefer TTL loupes as the field of view is generally larger than flip up loupes.
As for brand. That is totally subjective. I have used Designs for Vision, Orascoptic, and Surgitel. I have residents that use all of those and one that really like Zeiss loupes. Try as many as you can before you make a decision. Try on different styles, etc... Each company will give you their selling points, but it really boils down to what you like the best.