Q: Please advise which make and model of door closer would be most suitable for a regularly used internal office fire door?

A: Depending upon the traffic experienced by this door AND the type of seals (fire & smoke, acoustic etc?) the best and most adjustable door closer for this situation is the Geze TS4000 which is rated for a million cycles.  If the door simply has intumescent strip (no brushes), then a Geze TS2000 or even a Geze TS1500 (rated to half a million cycles) will be perfectly satisfactory and give good service.