Have a look at the high end electronic organs, they are pretty good (and also pretty expensive, but still a small fraction of what the real thing would cost). Mixtuur, Johannus, van der Poel and quite a few others will give you the opportunity to play and practice to your hearts content and typically a 50 euro donation to the church will get you a 1/2 hour (sometimes more) to play on the real thing if and when you feel you are ready for that.

On the cheap end: you can build your own, which is what I did, I converted a really old analogue organ that I bought for next to nothing into a sample based organ running Grand Orgue, it's not quite Hauptwerk but it is very good and sounds amazing, much more organ than I'm worthy of. Total cost: about 500 euros.