The tacho wire u can get the signal from the 30et loom, there will be plug going from the ECU loom towards the dash this is where the dash gets all its signals from, so with a wiring diagram yes you could.
Speedo loom will be seperate to the ECU loom i would think, should have things like alternator, temp etc, this should be easily swapped to the 30et.
Lights etc should be seperate? Or do they run through the ECU loom? If so your option is to remove them from the 20det loom and splice them into the 30et loom so they can gp through the firewall with the 30et ECU loom.
Cant really guess a price on the cost, everycar is different, both being common engine's if you searched could probably come up with a wiring diagram that would help.