Hi, I was in Porto for 2 weeks last August. It was warm and sunny for the most part, but the first days we were there we were actually really cold and wearing all of our layers, especially at night. Weather can be unpredictable but the average high for Porto is around 25 degrees in July. There are usually nice breezes up the river from the sea. We found that staying directly in the sun was hot but otherwise the weather was warm and nice, not too hot or humid. But of course that depends on many factors. When you are there it could happen to be 35 or 14...
It is becoming a popular tourist destination and some areas can be crowded but it is easy to find a spot to relax. Of all the busy European cities Porto is probably one of the least crowded I have been to.
We did notice that popular restaurants tended to be fully booked, but that was really the only issue as far as overcrowding.
Since you mentioned that walking up and down steps is a problem in the heat, you definitely should know that Porto is very hilly. However, it is possible to get around by public transit and taxi if you want to avoid the hills. Just plan accordingly and book your lodgings right next to the river where it is flat, then plan to grab a taxi when you need to go uphill. Or stay elsewhere and just grab a taxi to get around. Or take it slow when trudging uphill :)
Porto is AWESOME. One of my favorite places.