Lunita, I think you've voiced the same concerns we all had when we started riding outside, clipless or not. (I started riding again last Aug.) What I learned from these ladies is... find a bike that is sized correctly for you. It doesn't have to be an expensive bike, just one that fits you and what you want to use it for. I do encourage you however to get the best one you can afford to start with because the nicer bike is going to encourage you to keep riding.
If you want to ride trails and roads, then maybe a hybrid is the right type for you. If you are only interested in riding on the road, then maybe that is the type for you. Body mechanics have alot to do with how people pick the type of bike they buy. If you can't physically handle the more prone position of being on a road bike, then obviously that is not the style for you.
Also, I love my SPD's. But like most people, I started with flat pedals and stayed with them until I was comfy riding my bike and being out on the road. Then when I went clipless, I practiced in the house and in the yard clipping in and out before I got out on the road. Also, SPD's can be adjusted for tension so have the tension of the clips set softer so you dont have to struggle with clipping out. It's just a small screw on the pedal. It will still hold your feet in securely while pedalling but you will be able to snap right out when you need to. For flat pedals, I do encourage you to ride with very stiff soled shoes so that you have more support for your feet and get more power when you are pedalling.
As for spinning classes, I have never done them. I either ride on my trainer at home or I'm out on the road/trails with my bikes. I would assume one could use the same shoes as you do for your bike as long as the cleats fit, but I will let the more experienced ladies here tackle that one.

I've learned a lot from these ladies. Feel free to go back and read older threads, use your search button and look up what has already been covered. There is a great deal of knowledge to be gotten from this site. Good luck and welcome! I hope to see you around the forum regularly.