Touchpad (Mouse) Replacement/RAM (Memory) Issues

Hi, I have a Dell Inspiron 15R SE 7520. My warranty has just run out and how convenient my Touchpad has stopped working that's typical. So I'm wondering where I can get a replacement touchpad for my Dell and how can I install it? I think I got a dud laptop because when I first got it, the backlit keyboard wasn't even plugged in, and I've had endless problems with it. However despite all the problems, Dell has done a great job with the customer service and helping me out, so I thank Dell for that.

       And one other thing when I log onto the Dell website it's telling me I've only got 36.3% memory. Why is this? how come it's not 100%.  RAM doesn't just degrade that fast. 

So please help me out.

Thanks kind regards Angus