Assuming you drive the car ~12,000 miles a year and that you're actually going to follow through with an M3 in 18 months, you need to ask yourself the following: is it worth $0.10/mile for the next year and a half to have these mods? Since, at least in my book, their existence on a prospective car I'm purchasing is only going to take away from the value of said car (and certainly not going to add to it).
As such, I agree with the others. If you are actually keeping the car quite a bit longer, this could be reasonable. If not, keep it in good mechanical shape and unload it for an M3 $1,700 sooner.