I'm partal if you go carberated I would go with a yfz. with the cam mod there are not to many mild mod 450 that can touch it. as far as growing into it they make drop pegs and +2 steering stems to acomidate taller riders I'm 6'5 I under stand your 15 but if your going to be that tall you might even be cramped alittle on a rapter.
if you fuel injected I would prob. go with an ltr they have great power mild modded (efi cont. pipe and lid off) your would just have to test them and see what you think like I said i'm partal the carb. yfz are easy to get cheep hp. out of

 

Sweet looking ride desertviper when you say old ? You mean sold? Or crunched up? And yea you are right 4 strokes are great, but the hp to weight ratio with a (built correct) 2 stroke is just phenomenal. My motto is (if its fast there must be oil in the gas)

 

I guess the big question is, how much are you willing to spend on the initial purchase, and how much are you willing to spend on the mods?

Good luck with whatever you choose, and be sure to get some pics too.

 

It was a 2007 YFZ 450 that I sold for $5500, the same price I paid for a brand new 2009 YFZ 450 yesterday.

And on the two stroke vs four stroke thing, I hear what you're saying. I raced CR 500 dirt bikes in the desert for more than a decade when I was younger. My oldest son now has a KTM 300 XC two stroke dirt bike. I had a KTM 525 four stroke and could not beat him in drag races. I bought a dual sport bike, which was a KTM 950 super enduro, and drag raced him with that bike. On dirt the two bikes were dead even up to about 70 mph, then the 950 would leave the 300 like it was stopped. The 300 had a top speed of 80, and the 950 had a top speed of at least 140. I've always been impressed with the 300cc two strokes ability to hang right beside a 950cc four stroke, at least up to its top speed.

 

All the 450s have the manufacturers 450 motocross race motors in them, so there all fast.

Maybe the KTM maybe the quickest since its there 450 was the quickest in the 450 Motocross shootout.

2010 KTM 450 SX-F Comparison - Motorcycle USA

 

Yamaha lt400 may be right for you. This thing is a beast!

 

DId you pay cash to get that deal of 1000 bucks off the 09 yfz desert viper?.Im looking for a good woods quad not sure wether to go get a used yfz 450,Banshee or a brute force 650.I love the rip of the Shee but I know they run hot in the woods and when you blow them up its like 1500 bucks right off the bat to rebuild them.Id buy a 250R but they want more money then the newer 4 strokes.