info here:
http://homepage.mac.com/mattdenton/phot ... 400sl.html
manual here:
http://www.butkus.org/chinon/vivitar_40 ... _400sl.htm
For the day it was an OK camera. I had one when it first came out. Shot with it for about 4 years before moving up to a Pentax ME Super...
It was a little clunky and had a lot of vibration from the mirror flip up but it was not a bad camera.
M42 is the old screw mount used by Pentax so a lot of good quality cheap glass is available. Mostly from eBay (search for M42 mount). Also it is pretty easy to get an adapter so you can connect to telescopes.
Any non-dedicated flash will work. Vivitar 283/285, Sunpak 383 would be high end for this camera. Any camera store should be able to get you a simple non-dedicated flash for less than $35 (Ritz Quantaray QA-25 etc)
It's not a bad camera, has the advantage of being able to shoot even if the batteries are dead. Totally manual operation so you will learn a lot about photography with it, with only a minimal investment.
Pick up a 28mm, 50mm and 110mm lens and you will be set.