I would not recommend the YFZ450 as a first bike, they are a race ready quad which requires to be maintained like a race quad ,the 5 valve needs checked all the time even I have held back buying one because of the this I don’t like the idea of working on the bike after every few rides. The YFZ450 only has been out for 3 years with a RRP$12990 you will not find any near the 6k mark (if you do it will be a thrashed ex racer)

I would recommend the Yamaha Raptor YFZ660R, Honda TRX400 or the Suzuki LTZ400 these will be more beginner orientated and are not to far off the performance of the YFZ450 but I am sure you will have to go just over your 6k budget for these.

Don't also forget the factor in what other costs involved in owning one a helmet, boots, gloves, MX pants and jersey etc this is going to cost you around 1k plus you need transport (trailer?) and mainteince products.

Importantly bring someone who knows a fair amount about bikes along when looking at 2nd hand bikes if your new to bikes, you don’t get service history with these as you do with cars I learnt the hard way when I bought my first quad over 10 years ago. It would also be a good idea to get proof of ownership from the seller or check the VIN number as heaps of off road vehicles get stolen.

Feel free to PM me if you have any questions