Welcome. First, I don't use XM so I wont comment on that.
Your second question you need to consider yourself. How do you want it installed? Do you want everything to fit inside your dash, under your seats, in your trunk... How much room do you have? What type of pc are your planning on using?
Third, it is not hard to learn to install a pc in a car. You don't need to know much about the inner workings of a car's electronics to do so.
Lastly, this interface you refer too is called a front end. There are many available for you to choose from. There is an entire section on this forum just for front ends. I suggest reading through those posts and picking one. Then after you get your pc installed, you can worry about writing your own program.
P.S. You should read through more of these forums before posting such open ended questions. Check out the show off your project section for install ideas.