Well, you may want to do a separate post in the optics section to get some tripod advice or help with your decision.....by no means am I an expert on the subject.
When you say minimum weight to support an 80mm spotter, do you mean the weight the tripod itself is rated to support, or how much the tripod itself weighs....most 80mm spotting scopes weigh somewhere around 4lbs, and depending on the head on the tripod another 1lb or so. Then if you attach anymore things like a camera for digiscoping that's a little more weight to support. So I would think you need to consider all that when looking at how much weight a tripod is rated to support. As long as it meets your weight requirements i think you should be fine, and obviously some are still more stable than others, i would imagine the leg thickness or diameter when extended can play a role i this....and thats when it comes down to looking at reviews and talking to people in making your final decision.
As far as the weight of the tripod itself, my primary use is backpacking so im going for as light and as stable as possible......while still having a relatively short minimum height for putting in my pack, my current one compresses to 24" and my new one will be 15", for me, 24" was just too long for how I use it. Weight of the tripod I would imagine when not packing the heavier it is the more stable to some extent, but I wouldnt be buying an overweight tripod if using it for backpacking. I would say 3 to 3.6lbs or so is average, and the price starts going up as you start to go sub 3lbs. If you only occasionally use it for packing i would think there is nothing wrong with the 3 to 3.6lb average or so, unless you are extremely weight conscious. Now one benefit about buying a tripod that is rated to support more weight than just your scope, is the ability to hang your pack or bow or something off of the hook on the bottom of the center column (not all tripods have this hook). This will help for more stability and less shaking on windy days.
This is me completely talking out my ass here, just my opinions from my limited experience and reading and researching to choose a new tripod. I do recommend making a separate post in optics and taking advice from others as well.....but it will be more helpful for them to give you advice if you can specify to a certain extent what you think you're looking for out of a tripod.....max height, min height, backpacking or not, etc
Hope that helps a little!
