Advice on narrowing my used DSLR choices

[OpheliaH]

Okay, as a high-school student I don't have much money to spend and have finally saved up to buy my first entry level-ish DSLR.
I've been vacillating between these two cameras over the past week, doing neurotic things like creating spec tables and documenting reviews.
So. I've decided to join a forum and post up my question because I'm too indecisive about these things.

My choices are between Pentax K200D and the Canon 1000D, both released in 2008.
They're both pretty similar in terms of specifications, though the K200D appears marginally better.
Whilst the K200D about 100 dollars cheaper than 1000D ($250), what makes me reconsider is that it isn't Nikon/Canon and it may be harder to find lenses. Though, would backwards compatibility make it less disadvantageous? I always had the impression that it benefited those who already had Pentax lenses than those starting fresh. Also if I get stuck into Pentax, I wouldn't be able to move towards a full frame eventually.

A third option would be get the more recent 2011 Nikon D5100, but at over double the price I'd need to ask my parents to pay the rest of the cost, which I feel guilty by doing. I just don't know whether I'd outgrow the older cameras more quickly and that extra few hundreds are worth it. How outdated would the '08 cameras be for one looking to improve skill? Also, the prices of new lenses for Nikon are relatively more expensive than my other choices, but is it the same case in the used market? Should that even be a point of consideration if I just purchase one or possibly two used lenses a year?

Also, anyone have any particular tips on what to look for when buying used?

 

[KmH]

Canon is less backwards compatible than Pentax, because Canon changed their lens mount in 1987.

Pentax introduced their K-mount in 1975.