What if you paid cash for a van and saved the $150 a month for any possible repairs? You wouldn't be throwing money away on interest charged then. Really with any used car you could have to do repairs wether you spend $3k or $7k. At least if you don't have a payment you can then afford the repairs.
The book Taking Charge of your Fertilty is great. Thru charting your temp and paying attention to the signs your body is giving you, you can tell exactly when you can and cannot get pregnant. This is the most natural birth control I know of.