Considering getting a vita, is the wifi/3G model with killzone mercenary for $269 AUD a good price?
What sized memory card would be best if there is only around 5 or 6 games I plan on buying?
It's a reasonable price. Unless you find a sale you won't get much cheaper. Don't bother with a 3G model, you are better off tethering your phone internet if you want mobile online play. How many games do you plan on buying digitally? If it's all of them, then how many are you happy with keeping on your Vita at any one point? If you want to keep them all you will want to look at a 16-32gb card. If you plan on going physical games you can get away with a 4-8gb card with a little extra space for DD only games.
Edit: another top of the page!
Did you buy Wipeout 2048 + HD/Fury yet?