It's no too difficult. The hardest part would be getting the door skins off. You should watch some youtube videos, and enlist the help of friends. I would never let an "installer" touch my car. Maybe if you find a small shop / garage operation. But never Bestbuy, etc.
The small interior space of the smart works well, without a lot of amps. If you want simple rear speakers, you can just staple hook side velcro to the bottom of wedge 6x9 boxes, and stick them to the carpet. You don't need a lot of power. Go for the Nav or dvd head unit you want.
Where are you at in STL? I lived on Cherokee a couple blocks north of the old Lemp brewery.
I'm in Maplewood. My smart has fabric covered dash and doesn't look like most iv seen on YouTube. If I got a nav system made for the smart would i be able to install it or would I need help. I can take pictures if it helps