I imagine the engine itself would drop straight in.
As you probably suspect, its the wiring that'll cause the migraines. Having said that, its pretty old school so it might be quite straight forward. Theres an article in Practical Performance Car this month about a guy transplanting the engine from an Omega into a Scimitar. Apparently he ran the Omega and just snipped at wires till the engine stalled! An interesting approach to trial and error.
The best idea would be to remove everything right up to the fuse box, and perhaps even transplant that too. Compare it with your own, If they are the same just with different things missing or added, it may just be a plug and play affair, but be very gentle with all the electrics and take your time. Do you have another daily driver? It will take you about three times longer to complete the job than you originally anticipate! Make sure you fit a new clutch while everything is out, and have a good supply of gaskets. May even be prudent to do the cambelt and water pump too if theres no history for that.