No question: BMW 5-series or 7-series. The 7-series is longer and thus more comfortable and has a powerful V-8. Most of the 5-series have powerful inline 6 cylinder motors but some have V8s, i.e., 540 or 545. For $40,000 you'll have to buy used, but only a few years old, or use that as a downpayment.
I recently rented a big 2004 Cadillac but my '93 BMW 740iL is waaaay better but you'll want something newer. You won't be disappointed in a big BMW.
www.autotrader.com is a good source. Also, ebay has great deals, especially if you live near New York City or south Florida so you can look at the car before you buy.
The person is a YOUNG attorney, not a grandparent.
M5 is the only way to go.
If you offered me 7 assorted Lexis or Vettes or Mercedes over the course of a week, I STILL wouldn't give up the M5. And the owners of all those 7 wouldn't want to give it back to me at the end of the day, so...I guess that plan won't work. ;^)
Excellent choices. Either would impress your clients. However, a good pre-owned 7-series will eat up all of your 40 grand, or more. I would recommend a pre-owned 5-series. My wife owns a 2001 525iA, and it is a tight, solid vehicle, with one of the best rides imaginable. Check out this 525i in NM.