I know I have come into this discussion reasonably late but having had a number of E36 and E46 M3's over the past 12yrs I thought I might be able to help out with some advice.
Firstly the price for that particular car with those km's is far too high and I would suggest looking outside of Canberra for a car. I am in WA but have found the best examples always seem to come out of Victoria and price wise they tend to be very competitive.
Although all of my cars end up modified to some extent I like to be the one who does it. Try to find yourself a completely stock car to start with and complete any performance modifactions yourself so you know exactly what has gone on. With the S54 engine use only the very best performance parts available from reliable BMW tuners such as Turner Motorsport. If you cant afford to do a mod properly dont cut corners and use inferior parts, save the extra coin and do it properly.
There CAN be a number of issues with the E46 other than those you have mentioned although if you buy a stock car that has been well maintained and cared for by a specialist (forget BMW dealers) you should not find any issues with the rear subframe or Vanos system. The Vanos system is generally only suject to failure if the vehicle has not been maintained properly. In saying that I would suggest with any E46 you buy to replace the Vanos seals etc with the kit available from Turner Motorsport in the US. If you are mechanically minded and half handy with tools it's not a complicated job if you have the Bentley (publisher) E46 repair manual and the correct BMW tools which can be bought again from Turner Motorsport. I have rebuilt a number of non M car Vanos systems and all of those on my own M cars with no problems at all and I only do my cars so that I know that whilst I own there will be no drama with it.
The rear subframe fails on E46's through driver abuse. There is a huge difference between driving a car quickly and thrashing it. Of all the rear subframe issues I have come across it has come down to the tool driving the car incorrectly or belting it off curbs around a race track. There is a strengthening kit available but it's a job I would recommend you hand over to a suspension professional. It's worthwhile doing if you plan to track your car.
There are other issues with the E46 such as the cooling system and SMG issues. The SMG can cause major problems and expense if its been abused or not maintained correctly. I hope some of this advice helps and I am happy to expand on any of it if you'd like more explaination.
I have completed a ground up nut and bolt rebuild on an E46 M3 so there's not much I haven't learnt about them to this point and they are a relatively easy car to work on and maintain if you have half a clue.