I too recommend the Spec. Roubaix for the riding you describe.
I'd recommend SPD style pedals and shoes...you can walk in them while getting a bit more pedaling efficiency.
Tight clothes won't make you faster but you may look funnier in them if you're not slim.
Riding in the drops at 12mph or a bit faster does not improve aerodynamics so why bother.
You may also want to look at a gravel bike perhaps with flat bars. You riding description really points towards a bike of this type. You can ride on a variety of road surfaces, you are more upright, the ride can be more comfortable...you will only get faster if you ride harder to get stronger physically and aerobically. The bike provides minimal increases in performance...it's all about the body and mind at your stage of ability.
Good luck and keep us informed.