Hi Vaughan,
Despite one comment here on the list, using as SSD will make no virtually no noticeable difference to lilypond. It is not disk IO bound, and we are not talking about reading gigabytes of files. You may want an SSD for general reasons, but in terms of pure lilypond, having the fastest CPU you can afford is the best strategy.
If buying older equipment, realise that more recent i5 processors outperform older i7 processors by a lot, such have been the engineering improvements - don't just go by the name and number of the CPU chip.
Andrew