PS: I would be using the 450/515 as a dual sport mount. And you tech guys out there.............. your input is very important to me. Please render your knowledge.
Fred i have a 08 450 i went for the 450 and glad i did, i have also used my bike for dual sport across the middle of australia in the dessert approx in your terms 1500 miles on one oil change and it did not problems.
The 450 is a better bike IMO
The 515 power is not as smooth as the 450, the 515 has a very powerful bottom end and mid range but falls off in the top end, this is due to the overly square bore design (large bore, short sroke) the 450 will still blow your drz away and it got power every where in the range and is more manageable and smoother in the tighter stuff.
The 450 is better on fuel on a standard tank DS riding you will get about 130kms / 75 miles unless you get a larger tank, clarke or safari, i have got up to 20kms per litre just casual cruising /sight seeing etc thats 150kms/ 90miles to a tank.
With standard gearing the bike will do 95 to 100mph, fit a 14 tooth front and its still got heaps of power but will cruise at 70mph at 7200rpm, and plus 100mph top speed.
I have done 8000 miles on my bike , the only issue ive had was a blown head gasket, i replaced that and checked the valves and springs, piston and rings and big end, and nothing was replaced or for that matter any where near the seviceable limits, so i put it all back together and i should get another 8000 miles out of it, only the valves have had a shim adjustment once, i change the oil normally every 500miles.
The gasser is a great choice for DS its the closest race bike that can DS out of the lot of the 450's.
The 450 is the go, power, top speed, and still a great bike in the tight stuff.