Congrats on the engagement and upcoming wedding.
I can't speak for whether or not the salons you plan on going to need an appointment or not as I'm not from there, but from my dress shopping experience, most do.. but not all. They usually will say online whether or not they will require an appointment, if not you could always call. Weekends are busier for obvious reasons. I booked appointments on weekdays (took the day off) and the salons where pretty empty but apparently there weekends are nuts.
As for dresses, you better start looking. My dress will only take 4 months to get in, but some where 6 or even 8 months. Other then that, you will need to purchase a sample (off the rack) dress.
As for weight loss. You can always order a size smaller, but honestly, be sure you will loose the weight, its much easier to take a dress in then let a dress out, and you can generally alter a dress in a few sizes. I ordered a dress assuming I will need to take it in a bit. I was border sizes, and ordered the larger one just to be safe. The lady in the salon said more then not, brides think they will loose a lot of weight, order dresses smaller, and then aren't able to fit into the dress they way they want.. so be careful.