I wear baggie mtb 'double shorts' (inner short w/ chamois, outer baggy short) up to about 85 degrees, then switch to medium/long running shorts when it is hot.
Up top, wicking stuff (running shirts, mainly) on hot days, cotton tee shirts when it's not so hot.

If you are accustomed to SPD's, use 'em!
I personally feel really wierd riding on a bicycle in street shoes, but I've used cycling shoes and toe clips / clipless pedals since the late 70's. I couldn't imagine riding a bicycle more than a mile without them.