Definitely replace the missing kill switch. Use a Twin Air Filter, and Torco 80w. Yamalube is fine. If this were my bike I would do the following before the bike sees some time on the dirt. If your mechanical skills are decent all these suggestions should be easy to accomplish.
1.Purchase a compression tester and a shop service manual. Complete a compression test and compare the spec to the service limits. If out of spec, then the owner probably is fibbing on the top-end job. If the specs check out then you are good to go.
2. Take the chain off hold it up and see if it bends side to side. If it does replace it and the front and rear sprockets.
3. Grab to Front and rear wheel and see if you can move them side to side, and spin the wheels. If you have any stiction or play, your wheel bearings need to be replaced.
4. Tear down the bike.
5. With the engine out of the frame, check the clutch basket, springs and plates. Check the reeds for cracks or rounded edges.
4. Check the movement of all the rear suspension parts, grease those bearings with some quality Torco or Bel-Ray grease.
5. Go ahead and clean the carb.
6. Check your rear suspension and linkage for stiction and play. You have a 03 so the bearings should be ok, but use a quality grease like Torco or Bel-Ray and grease the bearings.
7. Grease your headset bearings.
8 If the forks were leaking oil, get them serviced. If the shock looks like it was leaking oil get it serviced as well.
9. Inspect the frame for cracks.
10. Inspect all your water hoses for wear. Look on the inside of the hoses. If they are white you need to inspect your water pump impeller and bearing for corrosion.
11. Put the bike back together, invest in some Myler's radiator flush, flush the cooling system Fill the Fluids.
12. Go ahead and lube the cables and flush the brakes, and you should enjoy your bike for a season.