A multimedia harddrive runs its own custom software embedded in a chip you cannot change that.
Programs you dowload need to be run through a x86/x64 processor to be intelligable meaning PC software only runs on a PC. You cannot just randomly get every software to work on anything electronic. You can't run World of Warcraft on your microwave for instance.
So for the laptop, check out the entire subforum dedicated to "frontends". Frontends are programs that do just as you want.
If you use a multimedia hard drive, you are stuck with what it comes with. Also a 15" lcd monitor is probably VGA. MHDD are usually just RCA out. And the only way to control those things is through IR remote control which can and will be annoying while driving.
Since you are planning to do a laptop part time, why not just do a laptop full time? Much simpler and more powerful.