Re: Installing 4.4.3 error
Yes you can download the factory image directly from Google. https://developers.google.com/android/nexus/images
As for flashing them its not very hard but it will wipe the data on your device using the factory images the way Google has the install script setup.
Since you already have the device rooted I'm assuming you have proper driver setup & communication between your computer and device. That makes things easier.
From there you need to download and install the basic tool package from the Android SDK so you can obtain fastboot and the necessary supporting files. http://dl.google.com/android/install....2-windows.exe
Once you have that downloaded and installed you do need to unzip/decompress the Google Factory Images you downloaded for the Nexus 5 and within each zip package there is another zip file within to unzip. Once everything is unzipped/decompressed you can follow the instructions found on the same page that you downloaded the Factory Images from to install the package.