I put your question to my husband, an experienced carpenter/cabinet designer and builder. He also tends to start talking very quickly when asked a question, so these thoughts and ideas will be a little scrambled as I took notes, but he's very knowledgeable. Here goes, in no particular order:
Use 6 x 6 for pergolas. For the fence, put posts in, put concrete, then using a skill saw and cut them off at the level you want because grade of ground varies.
Need skill saw because you're not cutting miters, all of your cuts will be straight, 90 degree cuts. Unless you're going to be doing inlay work, the skill saw is what you need. He says you probably already have one.
If you do have to cut all of your fence posts at the same height, pull a string and mark posts in between and cut them off at the string line.
TV shows love miter boxes because actors might hurt themselves with a skill saw.
(As he's talking, I remember using a skill saw years ago - it's more work than a miter saw, LOL.)
If you must get a miter box, don't get a saw with a blade more than 10" in diameter, because they are prohibitively expensive. Recommends Rigid brand if you must have a miter box. Good quality, but this a tool you hang onto, not sell. Does not need to be sliding, just a regular.
Then he got up and walked away, so that's all I got!