I think you shouldn't think of what kind of MIL you will make, but what kind of friend you will make. I honestly think if you take the viewpoint of "I'm gaining a daughter not losing a son" then things will be just find. In my experience (which I should say I don't have much of) the only time MILs get harsh is because they feel as if the "other person" has taken away their child. My sister doesn't have a very good relationship with her MIL because she married her "baby". That's not a good way to look at things.
I have the BEST relationship with my MIL. She's not just a good MIL, she's a good PERSON. She has the biggest heart out of everyone I've known. DH and I have gone through some ups and downs and she's always there to help us...whether its giving us some money to pay the bills, or giving us a trip to Disney World. I love her as if she were my own mother and I think of her as a mom and not a MIL.
As for my mom, its a little bit of a different situation. She lives with us

She moved in with us after my dad died. She takes care of the kids for me and does the cleaning and laundry, etc. Kinda like a free maid/nanny

DH gets along with her just fine though...sure there are some things that she does that gets on his nerves, but that can happen with anyone you live with. There are things that DH does that gets on my nerves.
Anyway, to make a long story short, I don't think you should think of yourself as an in-law...just think of yourself as a friend and all should work out just great.