For resale purposes, honestly, this is not the time to be stingy.
Personally, I estimate two failed and one good vet check into my purchase budget. A lady at my barn has had 5 horses fail the vet checks.
I strongly encourage doing the neck and back. I didn’t on my first horse and when something showed up in his neck when I was selling him, I definitely regretted it. He had to get an expensive bone scan and despite it being non-active, that seller passed. We then felt we had to disclose it to every other buyer that looked at him. Though he sold for a very good price, I wish I had done the xrays ahead of time.
For my money, xrays have one of the best return on investments in horses (which is to say, negative, but not crazy bad :yes:).