Mark, Another way to 'get into it' for Targa is to look at doing some Navigation. That way you can simultaneously build your more 'open class' machine now, drive it on the road, and as others have said - do hillclimbs, barbagallo, collie, the RAC Sprint Rounds (starting in late March) and so on.
That way you get to have a blast now, have fun with mates and importantly get some track time. You will be amazed how much you learn - and that is not to say you are not a good driver now - you may have talent - but the track time will teach you more than you think.
And while you're doing all that - sit in the passenger seat of a Targa car. You'll learn even more there. Its obviously much cheaper this way - but you'll still spend >$2k getting suit, helmet with intercom and all the other bits that go with it.
I've done 3x Targa West, Wrest Point, High Country and Targa Tas. It is no exaggeration to say it is far beyond any of the events mentioned above and takes a lot of planning and car development. (and a bit of guts to sit there doing 200km/hr through bends amongst trees on damp road) Its awesome and I really encourage you to aim for it, but do a couple of years of varied motorsport first. (yes Dad...)
And make sure its varied, don't just do Barbagallo for example, you need lots of different roads, speeds and corners.
When you do come to do Targa in a couple of years, you'll want a purpose built car for the job - so I reckon go mad on this one and have fun in the meantime.
If I can help with anything there let me know, I can put you in touch with Targa organisers who could possibly hook you up as a Nav.