I would revisit your previous iTunes setup and first ensure that the music has been ripped to a suitable format and bitrate.
Debate will rage as to where the compression of an audio file becomes audible, but I happily use 192 Kbps VBR AAC files without issue. I have these on my Macbook, but my Mac mini stores the 'master' rips of lossless ALAC files.
If you already have an iMac and iOS device, such as an iPhone or iPad, I cannot think of a more convenient way to do things.
To answer your questions...
1. The Unitiserve apparently rips bit-perfect to its hard drive. The Mac mini needs a USB driven optical reader to rip a CD. Even using a bit-perfect ripper, will I get the same quality rip?
Yes
2. The Unitiserve is more than twice the price of the Mac mini, but it is a piece of HiFi equipment, will I get equally good sound from either unit?
No reason why not.
3. I will want to use Hi Res files in the future. Am I better with the Mac mini to do this, or is it easy enough to get these files on to the Unitiserve?
I don't know. I have never and would never consider a Unitiserve, I'd be more inclined towards a Synology NAS.
4. My previous experience has really put me off WiFi streaming, so I think I'd prefer a wired connection. Am I right?
Depends on the quality of wireless connectivity in your house. Thick stone walls in cottages, for example, can play havoc with a wireless signal and a wireless extender would be needed. A wired connection obviously avoids any potential issue, but I have had none.
5. Finally, streamers. - Are they as good as a CD player £ for £?
Yes and theoretically better.