Adrian,
Yes it is possible for the costs to build up, but we are talking about a level of performance that exceeds that of cd players costing several thousands of pounds. As for the Katana, I have been put of somewhat by the messy board arrangement and the faff of so many power supplies to get it working at its best. The Katana appears from reviews to be a superb device, but just not for me.
Furthermore, I think that is would be very difficult to beat the performance of the Caiman SEG with the new Dorado power enhancer. The latest version also comes with the option of a power supply to match the Dorado. Having a separate DAC also gives you greater flexibility such as being able to connect a TV or Bluray player. A complete SEG setup with Dorado is under 300 quid I think.
Have you had the opportunity to hear an RPi based system? If not, I think you will be truly amazed at the SQ, even before messing around with fancy power supplies. There may be some AoS members near to you who could demo their setups.
I started off with an RPi Hifiberry DIGI+ which is still excellent. However, The Digione is better, though at 5 times the price of the former it should be. I no longer use the Hifiberry DIGI+ system so if you wanted to dip your toes in the "Digital Stream" so to speak I could let you have that for 50 quid plus postage. This would include the Rpi, Hifiberry DIGI+ board, a fully configured SD card with Volumio, in a black acrylic case. You would need you own USB power supply but this is effectively a plug and play system.
If I did sell the RPi/Digione it would be as a complete unit, again in its own acrylic case.
Whatever hardware solution you choose, I would strongly advise you get a free trial of Audivarna. This is not just a neat file management programme, but it also processes music files via upsampling and other "trickery" to greatly improve their sound quality.