First, you have to decide if you are gonna go with the existing motor or get a new one. I went with the more expensive option (new motor). I don't believe you can do a 131 with a stock case. If it is possible, the entegrity of the case would be in question.
The good thing about my option is that I still have an extra motor to tinker with. I can install it on something else or sale it and recoup some money.
Spending $4k will give you a nice build, no doubt, but if you wanna have more fun, you have to spend more money.. It all depends how deep in the game you wanna go... If you think JIMS is expensive, try R&R. They have just a crank that cost more than a lot of people's entire build!!