I renovated my entire place a few years ago, and the kitchen was the only job I didn't do myself..
We checked out flatpacks, bunnings, armstrongs, online, a heap of different DIY projects, and it was going to cost approximately $5K for the kitchen I wanted. Cheapest, possibly 4.5
However, we used a local kitchen company from the local paper and they came out to do a quote.. The guy was so helpful he even assisted in changing and improving my design.
The total cost, fully installed in ONE morning, was $6000 including a sink and tap and my pre-purchased appliances fitted as well.
Now in my opinion, that $1000 I spent to get everything done, saved me days of messing around and attempting to build my first ever flatpack kitchen that I had no idea where to start! Sure, they're supposed to be easy, but if my walls didn't line up to the right size how many hours would it have taken to get it right, find the flatpack to suit perfectly or trim the benchtops to fit, and how many hours reading instructions and construction and "ohh that's how it goes..lets go again" etc.
The company built it offsite and delivered it ready to install, and had it done in a few short hours. One benchtop was damaged (chipped corner) so they replaced it with a brand new one the next day. If I had of chipped it doing it myself, it would have stayed like that forever.
Before finalising your decision, get one or two kitchen places or cabinet makers out to make a quote, and then weigh up your options.
Cheers