What I'm talking about:
Modifying engine tuning so that the ECM programming will respond properly to boost. Typically, this at least means 'chipping,' unless you have an ECM that can simply be loaded with modified software. Then you have to worry about how good the software is versus the likely much more well-developed factory setup.
Worse case, this means a completely new engine management system, which can be a total whore to wire in without driving a lot of factory components insane. This may include replacement sensors.
Then there's finding room for the turbo, intercooler, and piping without upsetting anything else...properly modifying the exhaust manifold and downpipe (possibly other exhaust parts)...tapping into the right spots for your oil feed and return...modifying any intake connection that won't respond properly to boost as opposed to vacuum...keeping a functional PCV system...
Then there are things like...can the MAF and/or MAP still do their job with the increased airflow? Do you need bigger injectors or more fuel volume being fed to them?
I could kind of go on and on with 'what ifs.'
...basically, it's a lot more than just 'buy a turbo, intercooler, and some piping.'