I honestly can't imagine myself doing any sort of
serious gaming on a couch. Your back will probably not be on the right position and even if you get a shorter kinda of desk your knees will be hitting it or if they dont it will be too tall to be comfortable. Dunno if that makes sense
For the portability part you could make a mini/micro desktop built but peripherals will be hard to carry around. With that said you probably think, hey a laptop would then be better then. Myself i don't feel comfortable playing on a say 17" monitor while used on playing at 22"-24" but it you dont have that problem then you should probably look for laptop ^^.