Hello Apprentice :wave:
Welcome to MIMF.
Hey, great that you desire to further your keyboard skills in learning to play the organ. I can only echo the great advice given by CT64 in the post above.
You might consider also getting a tutor for the organ ... there are too many pitfalls that a beginner can fall into without knowing what is happening. The tutor knows these pitfalls and can really help the new student during the first years of study. I took 6 years of private piano lessons and another 8 years of private & college organ study, and all during those 14 cummulative years, I practiced, practiced and then practice some more ... about 3 hours minimal each and every day. It was a pain then, but looking back on it now, it was well worth all my efforts.
There is a book by John Stainer, Complete Organ Method that I would highly recommend. It doesn't cost an arm and a leg like the Gleason Method of Organ Playing, and the end results will be comparable.
Cheap organs ... under $3k? Be careful about what you are looking at. You are going to need a full 32 note concave pedalboard, and at least two identical 61 note manuals. It doesn't really matter what the sound is like as one never practices the sounds ... no, we practice the notes - the playing of the notes - the repetition of playing all the notes - never mind (for now) how the organ sounds or looks like.
Keep up posted on your progress ... and ask away any questions you have along your journey.