If you don t the rainwater will be dripping on your back all the time. Go stainless as the wind will blow sideways and flex the unit. Alum will be too weak. Make a good dry cockpit. Watch the height though as the boom will hit and that will wear out the bimini before its time.
All legs attach to the top of the coaming.
Personally like this a lot better. Would suggest you contact them for a quote, no matter which way you go, multiply their time to do the job by 2 however. The separate dodger, bimini and insert eliminates that problem. We are in Rochester, NY and mostly no one had them when we started but there are probably 6 or 7 of them now. They are wonderful obviously for keeping out the sun but also for sailing in the rain.