We used to operate a -100 and to be brutally honest, despite being a delight to fly (it is a pilots' forum after all) it was a parts magnet even 10 years ago. Getting parts is not easy and parts that actually work another matter!
I can't imagine there being any -100s converted to QC or freighter as this was usually done to a -200. For a more reliable option even the RJ series are getting on. An RJ70 would do the job far better.
Poor old things are worthless now for a reason. The 737 is more capable but even the early NGs are very tired and coming to the end of their useful lives.
If you can get hold of a tidy -600w on a cheap lease and get the engines chipped up you might have a platform to work with?
As said, if you're serious this is not the place. Speak to the manufacturers!