Just my opinion but i generally assume a laptop lifetime to be <3 years. Bits are harder to replace and spec is normally lower vs desktop. So for me i'd rather buy a cheapish one now and in 3 years time when its mostly on its last legs buy a new one which will be much better than if you have a 3yr old top spec one.
I have an Acer (not the best make by far) laptop running an i5 and 6gb ram and that easily copes with pretty much anything i throw at it. Running photoshop etc.
Built my parents a desktop computer a few years back running Ubuntu 3ghz triple core AMD processor, 8gb ram and that coped pretty well when i was doing some 3D animation with Blender. Although the CPU was the limiting factor for that as Blender eats it up when rendering the animation.
If you want it to last longer and work better don't put it on clothes/pillows/bedding or anything dusty. I have a padded lap tray if i want to use it on my lap. They like to vacuum up dust and then you wonder why it's red hot and won't open any programs.