The top of cupboard in the hallway outside the bathroom may actually provide the best solution. There is 50cm between the top of the cupboard and the ceiling. The top has a surface area of 60cm x 120cm
It should be possible to store a disassembled M605 in that space. You don't even need to take it as far apart as shown in the picture above. It should be possible to fit it into the storage space you describe in 3 simple steps:
* Plugging out the transformer
* Taking the head off the column (no tools required)
* Removing the column from the baseboard (this will require a set of spanners or equivalent to screw it loose).
You're then left with 4 modules which should fit in the space on top of your cupboard.
I personally have an M605, but I never took it further apart than described above - but I have the luxury of a permanent darkroom where it occupies a corner. I don't use it much these days as my Durst 138 is now my 'daily driver', but the M605 is a perfectly functional tool. If you get one, make sure that the transformer/power supply is included. Also check if the negative holder (I think this is the Durst 'sirioneg' if memory serves me) is there with a set of 6x6 masks (called 'sixma' and 'sivoma' - one is the lower mask and the other the upper half; both fit together into the negative holder with the negative in-between). I think there are also versions with AN glass, but I personally only have the glassless type and it works fine for me.
The M605 have came with a dichroic color head; I think those are the most common in Europe. Either this one will work fine for variable contrast printing, but e.g. a condensor head without dichroic/color filters will be fine too. You can always use the Ilford Multigrade below-the-lens filters. These are still available new and often pop up second hand as well. I never found a problem with using below the lens filters in terms of image quality. Some say/find that the Ilford filters do a better job at getting the highest contrast (grade 5) than what is achievable with the color head, but others disagree. YMMV.
Obviously, you want a lens to go with your enlarger; any decent 75mm lens will do. Most 2nd-hand enlargers come with at least one lens. The ones you come across may only come with a 50mm lens or a lens of doubtful quality, so it's possible that you'll have to find a lens separately. There are thousands being traded at any given time, so that shouldn't be a big problem.
In general, I'd say you don't want to buy enlarger parts separately, as pretty crazy prices are being asked for things like negative holders etc. There's a lively market for Durst components and sellers are very much aware of this. So usually it's a better deal if you can acquire a complete set from someone. The exception is the lens, which is more of a generic item and can usually be had at reasonable cost separate of other equipment.