Getting 200 out of an m42/m44 isnt going to be cheap.
Unfortunetly they came quite strangled from the factory so you would have to replace/upgrade these parts to see any worthwhile gains.
Cheapest option, and i hate saying this, is swapping in a straight six.
It all depends on what you consider "low budget", a guy came into the garage today where i work at part time, he had spent more on his intercooler setup then i had on my entire car!