Hmmmm mixed questions.. Tinny or runabout.? I think an aluminium runabout( see dhypes above) is the way to go for several reasons, mainly because they are more inclosed and can be set up better for more relaxed fishing -day and night . You can add radio, stereo, sounder etc and store things more or less under cover aposed to an open tinny. Also they are more appealing to the wife and family for a general day out boating . And lastly i think steering from the front is also 100% better safety wise, so much can be missed trying to steer from the back such as shallow water and other boats etc.
The harbour question ? Over 4 mtrs should be more than enough for the harbour BUT!!! a lot depends on where in the harbour and your experience. Around the harbour bridge area at busy times is a nightmare and more dangerous than the heads IMO regardless of your boat . Navigate carefully and venture out on good days and learn your boat and you should find a 4mtr will get u to anywhere within the harbour.
The $6 ish... thats a bad sort of price range ive found , you either get a top notch real old model for the $5K mark or you have to jump up to the $10K mark for the bottom end of newer type models. So depends if u want a not so awesome looking oldy and a goody(-5k) or a nicer modern one that u may have to spend some bucks and time on(+5k)