***** NOT A TEACHER *****
I was dithering about how to respond, but then I remembered a piece of advice I happened upon while reading Grammar Girl Presents the Ultimate Writing Guide for Students: When you can leave off the preposition and it won't change the meaning, leave it off.
If you insist on using 'with', you could say 'I do not have enough money with which to buy a car'. Here 'with' can't be omitted.
- For Teachers