Either option will work fine. Some considerations would be finance, diy capability, convenience.
I've read lots of good reports on converted beer kegs. They're 304 stainless steel, large capacity and do a terrific job as boilers - cut a hole in the top, add a tap at the bottom - use an outdoor gas burner and bob's your uncle. There's also been some discussion of this on the Aust. craftbrewing website.
On the downside, it can be very very expensive to purchase a new keg, and the Australian breweries don't want to sell used kegs. Sadly it's a crime to take an empty from outside a pub. I've read that sometimes you can pick one up at a scrapyard - I've even seen the odd one left out with the rubbish.
I've read that kegs can sometimes be obtained from pubs that import kegged beer from overseas - apparently it's not worth the cost of sending the empty back to the foreign brewery.
Alternatively you can purchase a large stockpot - Stainless steel or aluminium . (lots and lots and lots of debate on brewing lists concerning the relative merits of SS and aluminium). These are available at hospitality stores and restaurant supply outlets.
Another option is to boil in hdpe plastic using an immersion element - I wrote a post about this - think it's in the pub forum.
Even if you purchase a big stockpot, your stove may not have enough oomph to boil 20+ litres, so you may need an outdoor burner anyway, or an electric immersion element.
American Rubbermaid 5 and 10 gallon cylindrical coolers are very good but not inexpensive in Australia though they're cheap as chips in the US. There are plenty of false bottoms and screens designed with a rubbermaid in mind - so it can be easy to setup a system around one of these coolers. Can even buy a replacement bulkhead and valve (www.hoptech.com), or source the bits from local plumbing/hardware stores.
Alternatively you could use a 5 gallon plastic pail wrapped in insulating material. Or if you have an external heat source, you could use a big stock pot/keg etc and just monitor the temp.
Could also use a big esky and make a manifold. (John Palmer's online book contains for an esky manifold)
There are plastic ones (eg. Listermann) and steel ones (eg. ESB, www.morebeer.com). There are also manifolds (eg. Palmer) and screens. Take a look at Zymie's gear as an illustration.
Immersion or counterflow. Counterflow works faster but can cost more than an immersion (unless you make it yourself). Counterflow can be hooked up to a hopback - don't think an immersion can.
There's lots of off the shelf gear around but it's also possible to make a lot of gear yourself. So - I can't say if it's better to buy a system or make one.
Here are a couple of associated links
How to Brew With Two Small Pots on an Electric Stove
Want to get into all-grain brewing, but you can't afford big pots, fancy burners, your own grain mill, or even a temperature controlled fridge or freezer? Read on and see how one brewer has no shame in using the cheapest techniques possible to brew 5 gallons / 20 litres of yummy yummy homebrew.
Many homebrewers shy away from the idea of doing all-grain batches of beer because of all the extra equipment they think they might need. But the simple truth is that all-grain beer just tastes so much better than extract. Brewtopia Events Director, Owen Ogletree, is an award-winning homebrewer and certified beer judge who will tell extract brewers on this website how to make simple, 3 gallon, all-grain batches at home without a bunch of extra expense and equipment. Making 3 gallons of all-grain homebrew is very simple. Give these ideas a try, and we are sure you will find them fun and useful -- and you'll probably make the best beer you've ever had!