It's far too complicated even to consider doing it. A desktop PC case is more or less an industry standard but not so laptop cases. Your chances of finding a motherboard and case that match with all the ports on the board lining up with holes in the case are worse than winning the lottery.
You're lucky to have a large budget so a ready-made would be best. If such things as barebones laptops exist, I will be very surprised but that would be a way to go. I Googled it and found mainly Forum threads like this one. thanks for telling but are there any ready made laptops that have watercooling in them