I've got the double/single mount, antenna adapter and harness adapter for sale in steals and deals. I took out a pioneer Appradio 4, installed really nice. You won't find an aftermarket radio to fit the stock opening.
Installing a double-DIN is not hard on the smart. There are not only plastic adapters, but also harness adapters, so you don't have to cut up the car's wiring harness. I went with an Android-based GPS stereo.