Help with creating an invoice calculator with multiple drop down menus and functions

Ok so I posted a thread earlier regarding this but it didn't seem like I was very clear in what I wanted to do so let's try again.

Again, I'm pretty new at this and any help or direction is greatly appreciated.

So here's what I'm trying to do.

First you might wanna download my example file so you can follow along.

Looking at sheet 1 we have our first drop down; "category" in A4. I want to select an option there (Either Underlayment, Edge/Valley metal, Flashing, found on sheet 2) and have the next drop down (B4) automatically populate with different line items associated with their respective categories.

Take a look at sheet 2 and you'll know what I mean.

That's the easy part. Here's where it gets tricky for me...

After making a selection in the 2nd drop down I need a few different things to happen.

Looking at sheet 1 now:

1. Cell D4 needs to automatically fill in the Unit from its source (on sheet 2 E4)
2. Cell E4 needs to fill in the unit price (from sheet2 F4)
3. Cell F4 (Tax) needs to run a little equation. It needs to take the value from sheet 2 H4 multiplied by the tax rate (sheet 1 D1) times the entered quantity in sheet 1 C4
4.Cell G4 (Total) also needs to run an equation. It needs to multiply the entered quantity in sheet 1 C4, by the unit price (E4) and then add the tax value in F4

I need to be able to repeat this with all items and have each one show their different respective values.

Here's the next part:
I need to be able to start the process over in the row below for each different item that I have. I dont want to create a page full of menus. I want to be able to only show a menu when I need one. So if I enter 5 items, I only want 5 entries to show up, and it's blank if unused. Does that make sense?

Please help!