Hi Girls!
I am SO glad that I found this forum. I need a little bit of help

I am in my early 20's, and I have a bad back

I have 3 slipped discs blah blah blah from my military service. Since I no longer can run, I have decided to take up (and got the doc's OK) cycling to keep my figure trim, and keep my cardio endurance up. My husband road bikes as well- so I have a partner in crime

. Anyways, with me being brand new to this sport, I have a few questions about what kind of bike I need to get, size, comfort level, etc.
I am 5'8, and about 143 pounds. I know I can't get a road bike because of the decline to the handle bars, but is there a type of bike that I can ride that is somewhat quick that would still be comfortable? What size bike do I need? The terrain where I will be biking is coastal flat, with very little incline. Can you recommend anything? I would like to keep my budget fairly low, say less then $500 if possible.
Thank you so much!
L.