I'm 5'7", 170lbs, good shape, 46 y/o (this is starting to sound like a personal ad   ).  For all practical purposes I've never ridden before, although I did log some road miles on a bike I borrowed from a friend once when I was young and foolish.  I'll be taking my MSF course in early May.  I'm the cautious type, and I have no desire to "go fast", just looking to cruise back roads on the weekends.  I've been using the snowy (EXTREMELY snowy where I live) winter months to read my Proficient Motorcycling book and research bikes.

Given my lack of experience and height, would the Suzuki M50 (used, of course) be a ridiculously stupid choice as a first bike?  

50 ci is 800 cc. A good point to enter the fray.
A M50 is the old 800CC Intruder. A quick bike if you give it throttle. It has limited  luggage, and passenger seating, but is otherwise a nice ride for 1.

You are average height and weight, so, it's going to be a good fit.

Buy it used. They go down in value, and it makes sense to let the last owner pay for the "NEW TAX".  

HI;This bike would be a good choice. IT is lite easy to manover. Has good throttle responce. Enought power to get around traffic. It will cruise at 65-70 all day long. I have a C50T 05. I am 190 and 5'10" and 57 . This bike is quick enought for town and a great highway bike. I have had it to 100+ just to see how it handled.     The bike performed very well. When passing semi's the wind does't toss you around. It is very stable in heavy traffic. You willbe able to keep up with the big boys the 1300,s and up. I do. NO PROBLEM
I get 47-50 miles per gal. all round driving. This bike has been one of the best I have ever had. Low maintaince easy on tires. It is also easy to work on.
There are lots of after market stuff for them now.
Hope this helped... ..scruffy  

Seems like youv done some good research already if your askin about the M50. I too am looking for my first cruiser, after owning only one other bike, which was a few years ago now, being a kawasaki zzr600. What a superb bike that was, very quick. Im 32 now, and dont really want all the scream that goes with the sports 600's now, and looking at more chilled out riding. I myself have done some research, and the M50 heads the list, with only the Yam V-star custom giving it a challenge, but obviously these choices are based on my requirements. I will be using the bike to commute to work, which is only a 15 mile round trip, however i will have the occasional 45 mile round trip to see my folks. The V-stars finish really impresses me, and looks superior to the suzuki, however the muscle looks of the suzuki is very appealing. Also there doesnt seem to be much in it regarding fuel efficiency mpg, so its pretty much even and comfort also seems to be similar. Think its gonna be the M50 tho.....or is it ha!