I believe that if it is a true foreclosure being sold at an auction they will only accept cash (never purchased at an auction myself). Of course it doesn't have to be your cash, but traditional banks won't lend on a foreclosure AFAIK.
King County records are really great and easy to access online. If a property was purchased with a mortgage then there would typically be a record of the mortgage with the same or similar date as the purchase. I've only ever done this on a case-by-case basis, so I'm not sure if there is a way to easily look at a bunch of properties all at once. King County does have some data hubs that may allow you to download that data en masse. If not, there are 3rd party list companies that have that data.
I used to access the records by locating the property on Parcel Viewer (http://gismaps.kingcounty.gov/parcelviewer2/), but it looks like the behavior changed recently (still lots of good information, so take a look - once you locate and click on the property, click on Property Report then Property Detail).
Instead go directly to the records website (https://recordsearch.kingcounty.gov/LandmarkWeb/Home/Index) and search by the parcel number. That will bring up any deeds of trust letting you know if a mortgage was used to purchase the property.