The Lifehacker post is a bit outdated by now.
Below, my sugestion:
Get a Pi2 with 16GB+ SD card (8GB will not do when you want actual games on the SD card)
Get the extra's you need for the wireless/handheld stuff
2.1 a battery pack
2.2 get a USB controller to hack up into the handheld (PS3 - Xbox like controllers so you have enough buttons)
2.3 get a low power display to connect to your handheld build
- configure retropie (I.E. do like most people and get it working on a TV)
- make sure everything is stable
- try the same but from the display instead of the TV and make sure it works.
- try powering the pi from battery pack
- start hacking up the controller into a case for your handheld.
- curse yourself for ever undertaking step 6-7-8
If the above fails at the part where you move from set-top box to hand-held; you can get a bluetooth dongle and some wireless ps3 controllers and still use it as a console/mediaplayer.
Get a wifi dongle, so you can access the pi with smb/ssh (default enabled in retropi) this will help you loads if you don't want the thing plugged into an ethernet cable while working on it.
You can mount an NFS or smb dir with roms aswell, so you could easily have all your games on a NAS or something if you do run out of space.
note that you most likely will not be able to overclock the pi since the battery will run out quicker. so forget N64 games and some ps1 games.
Do know you are not threading an unwalked path here. Many people have made or atleast started this undertaking, like this guy.