Firstly, a note on the reason for choosing the M42.
Everyone's decision process is different and you'll have different factors which dictate the direction you go.
This is my only car so it's my daily driver and hence has to be reliable, economical, comfortable, and practical but also fun.
An engine swap to a more modern and powerful engine seems to fit in with this.
Of the options available, M20/M50/M52/M42/M44 etc, I chose the M42 because:
-It'll be much more powerful than the existing M10
-It'll be more economical than the M10
-Be as/more reliable than the M10 (not that it gave me any real trouble bar the oil consumption

)
-It should be easier/cheaper to install and run than the M52
-My insurance company will like it a LOT more than the 2.8 idea!
-It will allow me to use my exising gearbox and diff arrangement with the swap to the correct bell housing (i hope) see here:
http://www.bmwe21.net/forum/viewtopic.php?f=61&t=19851
Some unknowns for me:
-Sub frame requirement
-driveshaft requirement
-radiator
-I'm sure there's lots more I can't think of at the minute so any help is welcome!