The CD5i would be the most obvious match for the NAIT 5i. Secondhand ones are fairly easily available. Several very respected UK Naim dealers have a nice line in s/h offerings.
When you say ‘the different models’, if you mean the various CD5xxx ones, from the end user’s point of view, there’s nothing terribly significant to choose between a CD5i, a CD5i or a CD5si: functionality, looks and even price are all fairly similar. Just be aware that the CD5i (with the i in italics) is often called a CD5i-2, or even a CD5i Mk2) since just changing the font of a single letter in a component’s name is not the most helpful thing Naim could have done when naming their upgraded versions. Who’d have thought Naim wouldn’t be very good at naming things, eh? (I’m here all week).
Once you’re talking about other Naim CD players, there are significant differences, but those players would, arguably, not match as well with a NAIT 5i.