Lawn is frivolous. It is predicated on preventing HELL happening. If you've ever seen the exhausted, thin, parched soil under an old lawn, you'd know what HELL we're talking about. To prevent your neighbors sinning and driving fine SOIL to Hell pick up their green trash can's contents, compost it along with your own yard clippings. The resulting mulch will be spread thickly on your new garden.
Most people have jars, cans and bottles and baskets full of seeds from last year.
Also, their garden gave them unmanageable amounts of seed. These all surprisingly will plant up just fine. Forget the concept of hybrid varieties that can't replicate. They tell you that the germination rate of 2 year old seed is slightly less than fresh but if 9 out of l0 is now 8 out of ten, it's not a deal breaker. Won't put your nose out of joint.