Hey Kuhmo, so, you haven't mentioned whether you are looking to have your setup fitted by a specialist, or are you after advice as on how to do it yourself. Assuming your new H/U is a single DIN, fitting a new head unit yourself into a commodore, is more time consuming than difficult for a 1st time attempt.
There is a guide in the "How To Section" on fitted an aftermarket head unit , and theres a thread in the "car Audio Section" detailing a complete wiring diagram in which you can refer to.
Same goes with your front splits, You didn't mention what size they were, but almost any 6-6.5 inch speaker will be pretty much a direct drop in when it comes to replacing the factory ones. The bad part is the entire door trim must be removed, this means window winders , door lock tabs, and any electrics ect. So unless your confident with that, then defenitaley take it to a car audio centre and have them fit it for you.
Fitting price's will obviously vary from each store, as a rough guide, expect to pay up to $100, per part of the instalation, so H/U plus Front speakers = $200:thumbsup: