"very little riding experience"
That tells me you might not need to go too fast too soon. Could we make this a two-part process? Spend part of the loan money on a used 250 (I'm thinking Yamaha WR250R or Kawa KLX250S) or Suz DR-Z400 to spend a year or so on. Then use the rest of the loan money to move up and on once you've had a bit of experience.
And if I had those legs of yours, I'd probably look hard at the DR-Z because of the torquey motor. The 250's only weigh 20lbs or so less than the 400.
Edit: Forgot to mention: When you fall down, you've got to be able to pick it back up. Factor that in.