Welcome and congrats on getting an ST24. About the computer fitting, have a look on www.mirez.co.uk who has produced alot of useful guides. Surprised you not got wiring there as I have a 1999 model and it had only 5 wires for computer, earlier versions usually had the full 13 wires, which were as you were expecting plug n play.

From my experiance the earlier ST24s seem based on Ghia/X, until Ford realised how popular they were and started building the later ones more cheaply and to a price.

One thing worth checking especially as you have an earlier ST24, the water pump impellor, which on early ST24s had nasty habit of breaking up with obvious problems. You can get a metal impellor from motor factors for about £40. If yours has a black plastic impellor then best change it as these were the problem ones.

Enjoy your ST24 as you say it's a cracking car.