Yep, either will be fine. Unless you do a lot of continuous shooting at high burst rate, the Extreme IV probably won't be much advantage over Extreme III.
Don't buy Sandisk cards from eBay - most of them are fakes! Buy from a reputable dealer.
Just for clarification, Type I CF cards are the standard sort - 3.3mm thick. Type II cards are 5mm thick and are usually 'microdrives' - miniature hard drives. These were very popular when high capacity solid state CF cards were expensive, but have faded from the scene since prices dropped.