An Arduino USB Peripheral

I was drawn to an Arduino, when I was trying to tackle a problem with a Raspberry Pi equivalent. The problem was, how can I effectively minimize the amount of physical buttons I need. The current set-up is I have the microcomputer connected via RCA to a small monitor(no problem), I was also going to wire a smaller SPI screen to the computer and have a small program allowing me to have some preset commands and/or letters on the screen, and select them using a directional pad and a select button(problem). My problems arise when I realize that the screen is difficult to wire to the computer and stay independent. But then it hit me, with an Arduino, I already have documentation on how to wire this screen to it, I could just have the program on the Arduino. So my questions are few but important: A)How can I/Where is a good resource to find out how to make an Arduino a USB peripheral? B)The SPI screen has a built-in MicroSD slot, how can I make that a present resource in the Raspberry Pi(actually the CHIP from Kickstarter and Next Thing Co.)? C) What is the best model that would support this and have a low voltage requirement?

So after some research, I think the Uno is my only option, but its very power hungry. Any other ideas?

LordOfBurgers:
So after some research, I think the Uno is my only option, but its very power hungry. Any other ideas?

Come off it. The Raspberry Pi takes a current of over 500mA compared to the Uno's 30mA so you can hardly call it power hungry can you?

You can use one of the many 3V3 Arduinos if you want the voltage at the same level but there is simply no point, just plug the Arduino into the Pi's USB port.

I was unaware that it would accept power from the USB port, I thought I HAD to plug in the main power in, and as to the "power hungry" bit, I was referring not to the amps, but the volts, as I already have a need for at least 10v, I didn't like the picture of needing 15v when I'm looking to some time in the near future power this apparatus via batteries. I will look further at the Uno.

Power is the product of voltage and current. Power is not voltage. Do the maths an Arduino runs at a fraction of the Pi's power. Even so you need 5V to power the Pi don't you?

Where does 15V come into it?

When you use a smaller Arduino, like Pro Mini, you can run it on 3.3V, perfect for connecting it directly to a RasPi. And when you let it sleep until a button is pressed, it will consume only a few µA.

LordOfBurgers:
I was unaware that it would accept power from the USB port, I thought I HAD to plug in the main power in, and as to the "power hungry" bit, I was referring not to the amps, but the volts, as I already have a need for at least 10v, I didn't like the picture of needing 15v when I'm looking to some time in the near future power this apparatus via batteries. I will look further at the Uno.

Uno and many small Arduinos use a DIP or SMT version of ATmega328 chip. Max V is 5.5. best to not exceed 5V unless you like to replace a dead chip.

You can get or make Arduinos that run 3.3V. Those usually clock at 8MHz and use less power. The slower they run the less power they use even at 5V, simple as that.

Rugged Circuits makes at least one Arduino board that can take up to 12V signals, the Ruggeduino.
They also make external RAM for Arduino Mega boards.

Some Arduinos and compatibles use a USB/AVR chip. They can program as HID or CDC USB devices and communicate at full USB speed.

Well seeing as we have the "if" handled, how's about the "how"? Where would be the best places to look for instructions? I'm not(yet) fluent in the Arduino IDE, I have dabbled a bit, and am getting better, in python(not sure how much it'll help me, up to you guys). I have the screen that the Arduino will run, and a basic understanding of how to make a GUI, but that's about it. Where might a fellow go to quickly figure out how to not only make the program translate to the pi as a keyboard/mouse, but also have the MicroSD slot on the monitor act as mass storage for the pi?

The USB/AVR (and ARM) Arduinos can program as HID which looks to the host PC/RPi like a keyboard/mouse/joystick.

Sorry but IRL what you want is not quick and easy except to someone who's already made the same basic taco many times over.

If you're lucky you might find what you want or something that can be easily configured to do that once you learn how it works. If you're REAL lucky, that won't take long.

Otherwise you're asking for a lot more than simple help. You may get the help you need, but unless someone who knows it all well does it for you with your already prepared close to perfect spec, the whole quick part isn't going to happen.

You want a LOT and you want to avoid learning enough to DIY.

Quality. Cost, and Time have a hard relation in any real endeavor. "You want it by WHEN?" was a shop motto in one place I worked.