If you are looking for a pipe organ, find a used or a new small pipe organ with only 1-2 stops. They usually fit into a standard room.
If you are looking for an electronic organ (without pipes), then consider Allen, Johannus, and Rodgers among many others. They take even less space than the pipe organ.
I had this dilemma after my studies in the US whether to buy an electronic organ with lots of stops or a pipe organ with 2 stops.
I chose a real pipe organ with 2 stops on two manuals and pedals (from a local Lithuanian organ builder). But that's my personal choice. Other people might feel different about it.
But there is something about the mechanical organ touch and the real sound (like the ones Gene Bedient builds for home practice practice) which digital organs can't reproduce yet.
Perhaps the new ones are getting better but they are more expensive. But if you have a tight budget, electronic organs might be less expensive.
Then there are virtual organs (Hauptwerk) which are doing interesting things, too. You can play with the sound sets of real organs from around the world (famous cathedrals, historical organs) and pretend you are playing a famous Silbermann or Cavaille-Coll organ, among many other options they have available.
If we take money out of the question, then I guess the deciding factor for many people will be the idea itself: pipe organ vs electronic organ vs virtual organ.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my video Organ Practice Guide.