No car today should fail its first MOT providing you check the basics before you go i.e. lights, horn, wipers, washers, tyres. Saw a survey a while back that listed the reasons for failure, just listed them above, why don't people do simple checks before taking the car, just realised, they are muppets.
Since carrying out an MOT entails no work it simply cannot affect your warranty but should it happen to fail on say a suspension bush which is a warranty item you would have to take it to a Kia dealer to get the work done and then return to the original garage for a re-test which is a fair bit of hassle.
Since Kia's tend to be service every 12 months or mileage (depend on the age and engine) its odds on that the MOT will be due when a service is due (unless you do an annual mileage that makes your services due at odd times). Ours is 3 years old in September and will have the MOT while the service is being carried out. One visit and theoretically no hassle. I could save a tenner going to the garage up the road but is it worth the 2 hours or so of wasted time involved.