Am I missing something obvious? Help with mulch, please.

I am new to gardening outside of containers, and I could use some advice on next steps. My new home has some lovely garden beds which are unfortunately jam packed with invasive species. While hacking away like crazy at honeysuckle, buckthorn, callery pear, etc., we've generated quite a bit of brush and branches. Right now we are piling them onto a bed full of invasive ground cover to hopefully help smother the unwanted plants. We also tilled a bed and pulled up all the plants so now we just have a big pile of dirt. in one section. The plan is to buy some native transplants that will spread fairly aggressively. After planting them, what should I do to help prevent invasives from taking over (besides weeding like crazy)? It seems wasteful to put a bunch of organic matter on the curb in yard waste bags just to buy mulch. Is this the way? Also, is there a trick to getting the right type of mulch for a good price, or is it foolish to buy cheap mulch due to the odds of pesticides, seeds, and other contaminants?