when we moved into our house we had low water pressure as well. Instead of calling a plumber, I called our local water authority. The guy came out, checked everything and then said "yep - you have low water pressure" Duh - that's why I called you!. He showed me a valve that was in the basement that wasn't adjusted correctly and said "have a great day". I asked him what I owed him and he told me nothing! That's why you should always call the water authority first he said.
I suggest you do the same...it might be helpful to you too.