Honestly, you will likely get bored with the power of a Ninja 250. I had a lot of dirtbike experience, and I went with an EX500 (now called Ninja 500) for my first street bike. I think it was excellent all the way around, and I picked it up cheap (around $1.5k and it was perfect!). Another good one is the Suzuki GS500 - the older ones are called the GS500E the new ones with fairings are called the GS500F. I would get the older, simpler one for a first bike.

If you want a bike that's easy to learn on but you won't get bored with later, get a nice used stock Suzuki SV650. That is a great bike and once you get used to the power, start modding it with full pipes, clip ons, seat, etc. Those bikes are a blast no matter what your skill level. I would just say no matter what, get something you're ok with dropping once or twice - it will happen! Also, don't get in over your head with HP because it will catch you off guard at just the wrong moment.

The other good thing about buying a couple year old entry level bike is that you likely won't lose much if any money on your purchase. You can buy a 3-5 year old ninja for $2-$3k, ride it for a year, and probably sell it for the same. Not nearly as true for the bigger newer bikes that depreciate much quicker!

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2001 X5 4.4i
2002 Honda S2000