Need help for a begginner, simple project

Hi,

I'm building a Transport PC/HTPC for Hi-fi and I will be operating it with batteries for Sound quality reasons. My Pc uses two 12V DC inputs (one is for dirty power like HDD, CPU, DVD and the other is for clean like motherboard. This is cruital for Sound Quality). I will use two battery groups to power these inputs. Battery groups will consist of two batteries. One will be used by PC while the other is charged. What I need is to monitor the Voltage of the batteries that are in use and when they are close to deplate (%20 remaining) switch to the batteries that are in charge. Actually monitoring Voltage is not a perfect way of doing it. Is it possible to monitoring AH or WH? I am a noob at these stuff.

Switching is easy I guess. With a relay I can charge one battery while the other is in use and switch it to vice versa.

I also want arduino to to read from two different heat sensors, 4 different fan speeds and display them on an 16x4 screen. (My fans are 3 pin fans which will be used with a fan controller. The third pin, which carries the RPM data can be connected to arduino.

So what I need:

3 voltage or AH WH readings. (this is analog input I guess) 3 relay controls (analog output?) 4 fan speed reading (digital input?) 2 heat reading (digital input?) 1 16x4 LCD to show the readings

And If it is possible, I want to use Oled LCD. If it needs extra work to get it working then I can prefer normal LCD. If it is possible (if there is enough input/outputs) I can use arduino also for fan speed controller. At this situation it means 4 pots as input and 4 voltage outputs for fans.

Which arduino should I use?