I love blazers, and I think my count is around twenty now.
For shape, I think a lot of it depends on your shape and what is flattering for you. Stances and buttons and single VS double breasted totally depends on who's wearing the blazer!
For a good classic blazer, I think you should look for the following things:
1) A tailored fit. Make sure it fits in the shoulders! That's the most important thing to fit first. Then look at how it fits through the body and how the sleeves are in terms of length and width.
2) Go for a full sleeve length. That's the most classic to me. You can always roll them up for a more casual look.
3) Look for classic lapels. Notched are the most classic IMO, but peaked or shawl would not look dated either. Other things I would stay away from.
4) Avoid extras like ruffles and weird pockets and stuff like that if you want a really classic blazer that won't date.
5) Length kinda depends on your height and shape but I agree that just above hip length is generally safe.
I think you should go for one boyfriend blazer as well. It's proven to be quite a versatile shape that's stuck around after the fad faded. I like boyfriend blazers a lot with jeans and leggings as well as shorts and shorter skirts. It's also nice for a bit of a casual or menswear vibe.