By the sounds of it, with all the work you've had done wouldn't it be best to get the gearbox changed and get it back on the road. Better the devil you know than one you don't.
If you go and get another Omega, who's to say you don't end up having a host of problems to sort out on that one, not taking into account you probably wanting a WIM set up again!
And I've no doubt you'd be like this if you bought another car to find so many issues

You know what you've done on this one, so know what items / parts won't need replacing or fixing on it. Plenty of us on here who would be tempted to kick your rear into getting the gearbox replaced. Apart from Rob who seems to be having itchy finger syndrome with his spanners

All I'd say is, keep thinking about it and till you are 100% sure you know what you want to do.
I can understand you're probably pee'd off with it at the moment, but you've got support from us here if you stick with it
