I'm not familiar with the Naniwa kit specifically, but C-41 is normally a 100F (note F, not C) process. This isn't hard to achieve or maintain; just use a plastic tub filled with water at slightly above 100F as a water bath to raise the temperature of your working solutions, and put your developing tank in the water bath between agitations. You do have to be fairly precise with the temperatures, but not insanely so. IRC, Kodak recommends a deviation of no more than 1/2 a degree (or maybe it's 1/4 degree), but that's a conservative recommendation. If the temperature is a degree or two off, get it to 100F before developing the film.
As to keeping life, color developer doesn't have a long shelf life once mixed to working strength. If the Naniwa kit ships as concentrated liquids, though, it should keep longer in concentrate form, so long as you store it properly, so you can dilute as much as you need for one session and keep the rest in concentrated form. If the Naniwa kit ships as powders, the powdered form should keep for a long time, but mixing part of a powdered developer is inadvisable, so you might want to save up your film until you've got enough to use the whole kit.
