Martin at MEF Motorsport can supply both the ecu's and a loom.

And whilst the S40 will run the engine ok.....I would always urge to go for at least the S60. It's just a better ecu with more ability and also datalogging. The S40 really is an entry level unit.

But most places will supply an independent loom to run the engine only.

If you wanted it to plug into the existing loom you'd need to make a patch harness or re-terminate your existing loom to suit the DTA.
Obviously either of these routes can present their own challenges and you'd need all the info relating to the cars wiring and existing ecu pinouts. But in many ways it can be a nicer option.
On older cars it shouldnt be too difficult...more modern stuff can be more awkward.