Magento import products via XML

I have 4 XML files with product, pricing, stock and option data.
All data must be imported efficiently into the magento shop.

Subsequent calls of the import must update the data (compare for existent SKU).

There is no special documentation. Data is self explanatory. Match as many standard fields as possible.

Image URLs are provide in XML. Images must be downloaded and assigned to the products.
On subsequent calls images must not be downloaded again.

One file contains custom options that have to be imported too, as simple product dropdown options.

I provide only XML files. You have to set up own Magento dev environment and demo the implementation to me.

Further conditions:
- Must work with Magento 1.9.3.1
- Import must be runable via command-line
- Main logic must be in Magento extension, meeting usual extension standards (folder structure etc.)
- Import must be fast! (Coders using FastSimpleImport will have preference https://github.com/avstudnitz/AvS_FastSimpleImport , rather not use DataFlow)
- About 3.000 products
- Skript has two phases: Download and Import
Download-URLs must be configurable (Example files will be provided)
- Data must be treated fully confidentially
- Timeline: aprox. 2 weeks