When you get the car, dyno it and get a read out of a/f ratios. If it's too rich, SAFC.
Ask yourself, why do you want the RSM? Rice factor, or for the data it can give you, and do you know what it means?
If you intend on modifying the car more in future, get the PowerFC or something similar, and don't waste money on something you'll end up replacing, ie. SAFC.