mk1 or mk2 . mr2 can use mc21 crank with a bit of machining or there is a bloke in australia that can rebuild all nsr cranks . mc18 no longer available new only mc21 cranks still acailable .
I would say nsr250 on the road there cranks get 30000 - 40000 kms give or take . But it all depends how they are looked after for the life of them .
As harryredchow said 10000 out of a topend . I would do rings at around every 5000km and see what the pistons are like and if they need replacing as it is fairly cheap to do.