If your looking at a 6th gen, get an 06-09. They are very solid and have had most of the issues of early 6th gens fixed. That and they use a 4 wire obd connector so you can talk to the bike if needed. Many are available for 4k or under. As for 02 issues. Harness update/repair, r/r, stator, last time fork oil, engine oil, coolant, brake and clutch oil, tires, changed the chain and sprockets so wheel bearing, valve clearance, what minor mods have been done (some dont think desnorkle or flapper removal is a mod). The list could go on and on.
I dont know what adverts you guys look at but i did not find any nice 06 or newer for less then $5000. I literally bought the cheapest 6th gen i was able to find with a reasonable condition and i had to pick it up from NV. There was a bike shop in Norcal that was selling a rare 2009 all black low mile viffer for $7k. I liked that one because i think it was the only year with all black frame, swing arm and forks. Only reason why i didnt buy it is because i had a very low budget to work with lol.