The wiring loom in any car that had a variant within the model range equipped with a tacho should already have the wiring for the tacho incorporated in the loom, just not connected. This has always been the case in my experience anyway... form rollas to geminis, escorts, datsuns, commodores etc and all the later model stuff as well... whack the tacho in, find the right plug and away you go.
And yeah... even if it doesn't have the wiring, you only need 4 wires to make it work. Coil signal, power, earth, and illumination.
As far as your heater core goes, its essentially just a little radiator. I had a leaky one repaired in a VR commodore once and it cost me 50 odd bucks at a radiator workshop for the dude to find the holes and weld em up. If yours is corroded and beyond repair you would probably be better off adapting another one to fit rather than pay upwards of 400 snails for a new one. They're not terribly complicated devices and wouldn't be too difficult a job I would imagine.