Hi Michael,
As others have noted, it is best to work backwards from the maximum assumed exit pupil your eyes can accommodate when fully dark adapted.
The exit pupil is the diameter of the beam of light coming out of the eyepiece when used in that 'scope. The exit pupil is approximated simply by dividing the aperture in mm by the magnification.
Assuming "about average" dilation for the healthy eye it is about 6mm. If the 'scope eyepiece combo is producing an exit pupil over 6mm your own iris is vignetting the exit pupil and not all the light from the primary mirror is actually making it into your eye.
The aperture is 300mm and the desired maximum exit pupil is 6mm, then the minimum magnification is x50 before your eye begins to vignette the light beam coming out of the eyepiece.
So the maximum eyepiece focal length you can use in 12" f/5 without the exit pupil being vignetted is therefore 30mm (1500/50). You can of course go for a longer focal length but you will be wasting light.
Lots of different views expressed here on the cheaper end of the range. You expressed interest in a "Kellner". Don't go there -- ancient design, poor aberration control and a narrower field than an Orthoscopic or Plossl.
The around 30mm generic/Taiwanese/GSO etc "wide-field" offerings ... you get what you pay for. You can expect at f/5, that at least the outermost 40% of the field will suffer from quite yucky star-images from coma and off-axis astigmatism. These eyepieces actually perform pretty well in long focal length telescopes (like a Schmidt-Cassegrainian at f/10) but not nearly so well at f/5. If you feel you can personally tolerate that long-term, fine, but there's a lot of wisdom in the idea that investing in higher quality eyepieces is a good investment because you can keep it if and use with other 'scopes if you ever decide to part with the 12". A good eyepiece is a jewel forever.
The 68 degree and 82 degree offerings from premium manufacturers incorporate more glass elements and correct for the aberrations far better. They are are expensive but you will get a far larger well-corrected field of view and the view will be commensurately much nicer.
FWIW, nearly all my eyepieces are Televue Naglers. It took me about 15 years to acquire what I call a full set (of eight), but the patience and the extra dough was worth it.
I have also done (some years ago for S&T) a test report for the 28mm UWAN David (Astraltraveller) mentioned and it is a very, very nice eyepiece. The aberration control was on par with my own 26mm T5 Nagler and it is about 60% the sticker price.
The 28mm UWAN would give you x53.5 magnification and a true field of view of just over 1.5 degrees. That's a huge FOV.
I also own a 30mm Explore Scientific 82 deg and it is excellent. It loses very little compared to the venerated 31mm T5 Nagler in aberration control but is a lot, lot cheaper.The 30mm Exploration Optics 82 deg FOV in your 'scope gives a field almost 100 arc minutes across. Expansive!
Think about a pre-loved eyepiece -- they come up from time to time on the forum here and other forums as well.As you are in Victoria, have you been to any of the observing events down there? It might provide you the opportunity to test-drive eyepieces owned by others.
By way of indicative example, here is a second-hand 28mm UWAN for sale in the US on Cloudy Nights (you will need to join to view the ad --it's free) for $170 USD. Assuming the seller is happy to sell to you in Australia (not all but most are) $170- by the time you do the USD to AUD conversion, add $25 for shipping then add GST on being landed here, you can get out of this deal for about $300 AUD and get a top quality ep.
https://www.cloudynights.com/classif...8mm-2-82d-fov/
In the end I know, your wallet/budget will have much to say. Best of luck with your choice.
Best,
L.