After 1 year of researching, waiting and saving money, I was able to convert my Nikon D70s.
I did the conversion on my own, using an EdmunOptices 670nm IR filter.
Nikon D70s: 200€
Edmund Optics IR-filter: 8€
Still need to check out the auto-focus and everything else, but the camera works and gives me the IR-effect with minimal exposure time as expected and I can use the viewfinder =D