if dust is allowed to accumulate, the fan wont cool the gpu, gpu will go into heat throttle (poor performance). so a nice clean fan = no air cooling impedence.
the gpu will last a long time, probably a good number of years.
as Solandri maybe able to testify, having a top end gpu will mean that 'mundane' games will take so little power, that you wont notice much heat. If you get a poor gpu, it has to work harder for all the small stuff, so will heat up more often. decent client games will always use as much as the gpu is capable of (default defined detail settings) and its upto you to turn it down.
This issue of hot gpus from the top end is because the gpus are not 'full fat' they are semi-skimmed versions of the real deal. so they end up having to work a little harder than the desktop equiv. (generally denoted by an 'M' after the model number).
IMO, as you cannot upgrade a laptop, you should get the most powerful system you can afford. you dont want to use it for a year and then think: i cant play overwatch 2/DooM 2/BF 10 (as an example of future games - fictional).
but if you intend to only keep the laptop for 2-3 years go cheap, its going to be thrown away and replaced.