Coil Over vs Torsion Bar
LOL well as to "when" it needs to be done, that would be an individual issue. If you have deep pockets then one is not necessary. If on the other hand you have to budget for your build, then you probably need to do one earlier vice later. As far as whether the wife's buy in is a factor depends on your marriage I would think... :)
I do cost benefit analysis's on many big ticket buys; usually its more of a comparative analysis to determine what is a better value. I also do weighted factor decision matrix's to help me to make decisions especially when I know I am being too emotional about something (want something just because its cool). These are pretty subjective and you really have to try hard to keep emotion out of the process in order to arrive at a position that is accurate.