You first have to remember what you are looking for there mainly a battery powered setup of everything. A laptop is self contained with a 4hr. typical average use time before recharging. Without a builtin display you now have to battery power an external display of some type.
Not all portables will allow the input from external sources since they are simply self contained small tvx you can carry around. The lcd type tv come in all sizes while larger models usually expect external input from dvd players, analog and DVI sources with the newer ones out. But a 14", 15" desktop lcd monitor would also be awkward.
The thing you have to do is decide what will best fit into your budget as well as suit your project there. Finding a low cost lcd tv with allowances for external sources with battery capability will be one task since there are all nrands, kinds. and sizes to look over. Then you have to convert vga to S-Video or DVI depending on what you find.