Avoid laptops with a "u" model CPU (like the i5-5200u) and "h" model CPU (like the i5-4210h). Those CPUs are soldered into the motherboard.
A laptop that allows you to upgrade the GPU must have a MXM interface so that an MXM format GPU can be installed. You also need to check that the graphics card is not soldered into the MXM interface because I have read about some laptop manufacturers that actually do solder the GPU into the the MXM interface.