A much cheaper alternative using the Unimog, I did this for about 85,000 Credits.
I bought a Unimog off the Auction house for 55,000CR buy it now (trust me they do appear quite regularly at that price). If you're not too bothered just buy one for less than 100k, if its been tuned already then its a bonus.
With tuning I only upgraded it to Max class C 600 which only costs around 30k compared to the 400,000k plus using S1. The tune i used was called Cheap Power under the suggested tunes tab.
You can also upgrade it to class B quite cheaply. Just search for class B tunes, there are lots. It's probably a lot easier this way but its definitely possible at class C.
I used the Ambleside Danger zone for this too, you need to get to about 120 MPH which is fairly easy to do if you go back far enough. It'll take a few attempts, just use the rewind tool to get the best angle.
Straight after I sold my Unimog on the auction house making a bit of profit.
Note: This is much more difficult in winter, I recommend either waiting for another season or upgrading the unimog to a higher class (S1) if you don't mind spending A LOT more credits. At the time I was saving for the castle so I couldn't afford it.