Ordinary & necessary. Not lavish & extravagant.

I plan to attend a conference later this year that is related to my line of business/industry. Reading over the irs guidelines covering business travel expenses, you can't deduct expenses that are lavish or extravagant. But how is this determined? My budget is to stick around 1k - 1500 for all expenses. I don't plan on staying at a Waldorf Astoria or Ritz but have found some nice options that are within the budget. Those places just happen to be really nice since it's in Vegas.

Am I supposed stick to places like a motel 6? I'm also looking at places that aren't attached to casinos. Is there anything else related to business travel I need to know? Should I have a memo or something written discussing what was covered, how it will benefit the business and who I met and spoke with? Or is that overkill? What's the right answer here? Thanks for the feedback.