'13 kx250f valve advice.

The seats will have to be cut with either type of valves. Normally the exhaust valve guides are what wear the most.
They would have to be checked.
KW valves will last a very long time if installed correctly with new springs, retainers, keepers and seals with the proper head prep. The engine will not rev as fast with them, as they weigh more, but they will last a very long time.
The key to longer valve life on any modern 4 stroke engine is to not bounce off the rev limiter all the time.

Paw Paw

I went the stainless steel KW valve route on my 2010 kx250f. You will notice a small loss in over rev. But I was never one to rev my 250f hard. I'm now on a 450f, which suits me much better.

The valves will last a long time compared to the Ti valves. My head never got any special treatment. I spent many hours phoning and researching, and could not find a solid answer on whether to cut the seats or lap the valves. The local bike shop actually just dropped the new valves (with new springs etc) into place with the old seats and said they will be fine and will 'wear into place'.

I never liked the fact that they did this, but I never had any issues (i may have gotten lucky though).

I would follow Paw Paw's advice though, and have SS valves fitted properly with the seats cut.