Hi Suprella,
I live between Brisbane and the coast and bought my NB8B (late '00, which is the 2001 update) in March and helped a friend buy the same a few weeks later.
As others have said, summer is the one season that you don't really want a convertible.... it is hot and rains a lot

How that affects prices I'm not so sure.
I wanted a red one, but they seemed to come at around a $2k price premium for the standard models when I was searching, so I settled with midnight blue. Black seems to be pretty average so no problems there.
There is no need to settle for a car with high kms. In my opinion, cars in the 100-120k kms range are ideal; they have been dríven enough to get regular use and servicing, but aren't worn out. I bought a '00 NB8B single owner 6spd manual with 63k kms for $8k in Sydney. My friend ended up paying just over $7k in Brisbane for a '00 NB8B 6spd with 170k kms. In the 6 months we have owned them I have had to replace a soft top that fell apart after I bought it due to a lack of use.... a cost of a low kms car that had hardly been dríven in 3 years. Meanwhile my friend has just had to replace the suspension and all the rubber suspension bushings in the car because everything was so worn out that he couldn't get a proper wheel alignment done on it. There was a VERY obvious difference in how the two cars drove when we bought them.... mine was just newer. So you have the money... no need to settle for less.
Your budget is very generous for a facelift NB generation car ('00-'05). Early cars ('00-01.... don't buy a pre-facelift car) are the cheapest and you can easily get 120k kms for under $10k (more like $8k). The NB8C update that came mid 2002 sometimes saw a drop in rated power.... but there is a good chance that was just because of a new testing method. These later models also came with a few extra creature comforts like a low fuel warning light and boot light.... not game changing, but nice.
One thing I would like to suggest is investigating the factory turbocharged SE model of 2004. These will be at the upper range of your budget, but it is very easy to get them in red and if more power is something that you are likely to seek then it is a good base (even if you swap turbos later... it is beneficial for insurance to have factory turbo vs aftermarket).
My opinion is also to go private sale instead of dealer. You do get some mandated warranties and stuff with a dealer car, but you generally get NOTHING on the history of the car, and you don't get to sus out the owner. Also, people who can't look you in the eye and tell you you are buying a good car are the ones that get a used car salesman to lie it to you instead.
Also, I would strongly recommend not limiting your search to south-east Queensland. Buying from Sydney and Melbourne is often significantly cheaper (I have saved at least $2k each on my last 2 car purchases from Melbourne and Sydney respectively). A flight down is $120, petrol back is a couple of hundred tops and a night or two at some hotels on the way. You could choose to take a bit longer and find some spots to do some writing along the way! You could also have them shipped back for ~$700.
Good Luck

Also, I think Kirk still has his low kms (~70k kms?) black NB8C ('03 I think) for sale on the Gold Coast. It comes with a hard top as well if that is your sort of thing. It was a pretty stock car 6 months ago, was modified and then had the parts put back to stock again so he could buy a turbo NA MX5. Not ideal.... but it is a pretty honest car and owner. PM me if you can't find the ad.
Good luck
