Here's my two cents on the breeds: don't buy nubians or saanens. Nubians don't want to pack and saanens overheat. I have owned purebred saanens and hybrid saanens; none of them were as good as my hybrid alpines. I also wouldn't buy purebred alpines; they are very athletic, but they have high-strung personalities. IMO, the best option is a hybrid goat that his half alpine and half something else, such as Toggenburg or La Mancha. Ultimately, their attitude is more important than the breed, but if you stay away from nubians and saanens, you'll have a much better chance of success.
As for weight, the rough rule of thumb is 20% of their body weight for any 4-legged animal. I always tried to keep the weight between 25 - 40 lbs (in addition to the saddle weight). We loaded some goats heavier on elk packouts for short distances over relatively flat terrain, but that was the exception and not often.
In my experience, I found there were three grades of goats: those who wouldn't pack at all, those who could pack weight but needed regular exercise, and those who could pack without any exercise at all. Those who could pack without exercise weren't bigger, they simply had the attitude that they weren't going to get left behind no matter what. Some of my smallest goats were the best packers and vice versa.
My advice is to buy more than you need because they won't all be winners. Some will die early and some won't be great on the trail. On the plus side, they don't need any training like a horse or a llama. If you bottle feed them as babies, they'll follow you around like a dog and you just put a saddle on them and go.