A big factor is really whether the galaxies have an active or inactive black hole in the middle. ours is inactive as is andromeda so when we crash we wont notice. The chances of a 3 body inteaction with another star is so remote thats its virtually zero.
The problem is really about dust and gas. although its near vaccum densities , there is a lot of it...an awful lot. If gas clouds become gravitationally linked it will compress untill...like our sun ...it goes supernova and greates a new star.
If you are within a few million light years , you are in for a very bad day . Gamma ray bursts from the explosion will destroy pretty much everything organic at high levels. You'll get a nice suntan for a few milliseconds then you're toast.