Start with a bog-standard Uno. Most projects and examples are built around this, so you'll find it easy to learn. Once you are very comfortable with it, then likely your project could work best using a Mega as the controller.
A Mega, however, has some (minor) differences that - when you put a project designed for an Uno on it - that might trip you up. Mainly things like certain pin assignments are -slightly- different; also on a standard Mega, there is a particular pin that isn't properly brought out like it is on an Uno (see why something like the Nootropic Video Experimenter won't work on a standard Mega).
There is a possibility that your entire project could work fine with an Uno - depending on the number and type of I/O pins needed for all of the peripheral components - but you'll be able to figure this out best once you learn how the Arduino works with such parts. Take it one step at a time - learn how to get an LED to blink/dim first, then how to control it with a switch, then with a potentiometer, then with an LDR, then with other components - don't try to jump in all at once, and you'll have an easier time of it.
Also - as you learn with the Uno - you might think about the possibility of using port expander ICs or similar devices; these can allow you to gain more I/O pins (depending on what you need them for) at the expense of only a few pins on the Arduino (for the interface to the ICs - most use SPI) - but with the need for a custom interface library (which you may have to write yourself). This would allow you to use the simpler Uno - provided your memory requirements aren't too heavy.
Think of the Mega as something like a hammer - everything looks like a nail - whereas with a bit of thought and creativity, you can shoehorn in a smaller controller like that on the Uno, and it will meet your needs fine, and the possible expense of slightly more complex code and the need for more external interface hardware.
As far as kits go, I don't have a specific one in mind for use; if you understand electronics well enough (to know what the parts are, their values, etc) - I would look over each and every potential kit out there, make a list of what common parts between the kits are, then see if you can put together your own kit of parts for less money than buying the kit (sometimes you can - sometimes you can't). At a minimum, it would familiarize yourself with the kits, and give you an idea of what's available - some may have certain parts that are more applicable to the learning you need for your project than others.
Also - regarding a certain kit - I like the book for the Earthshine "Arduino Starter Kit" - which is available for free:
There used to be a complete kit available, but it doesn't look like it is available any longer (which is a pity); you might try to find a kit that has the same parts as listed in that book - that way, you could do the projects in the book with the parts.
I hope this helps. Good luck!