If you want a 'ready-built' solution then the Pi-Top may be the way to go.
If you want something more 'homemade' then I, along with many others, have used a spare LCD from a laptop, coupled with a cheap HDMI eBay driver that converts the Pi HDMI output to LVDS for the LCD (around £20-£30 GBP).
Add a wired or wireless keyboard (with additional trackpad) or mouse & keyboard combo (I actually just found this Pimoroni offering - it looks SWEET - I might get one)!!
Provide power (mains or battery, you'll need 5V @ 2.5A for the Pi (it WILL run on less but needs the extra overhead when you start to hammer it / drive peripherals - else it will 'brown out'.
I bought one of these for the Pi power requirements (2.4A - tested and it gets pretty damned close) - currently half price as Maplin is closing down. Mine was via Amazon - from 'Cash Convertors' of all places - (totally not nicked and chopped in for cash by some dodgy type . . . . . . . . )
The screen power requirements will depend on which you choose (my 17.5" monster + driver board needs 12V at around 800mA to be safe) - the official 7" screen takes it's power from the Pi. There's your power bank supply doing the whole 'shebang'.
Buy or build a case and you have a pretty cool laptop you made yourself!
