You can find the OVA and VMDK files in the virtual machines section of the LAMP product
If you use the OVA or the VMDK in your local machine, it will work in the same way of the Lightsail solution. Please note that we build our virtual machines and cloud images using the same pacakges
If you want to install Drupal on top of the LAMP VM you have in your local machine, you will need to access the console of the VM and then download and install the Bitnami Drupal module
Yes of course! It's completely valid. We provide OVAs in our site and you can use them with VirtualBox. That's fully supported and recommended.