I'm not going to talk any technical gubbins because as you can see above there's plenty of member here with far more knowledge than myself, but I would like to add my piece
Most people on here will tell you that owning your 75 is like no ownership experience you will have had before. It's not normal human behaviour to get sentimental about your every day car, especially when you're only spending the kind of money they go for these days, but you might find that this time you will...! I'd say take your time over choosing the car, if you can offord to wait a little. I think shifting from the Omega to the 75, you'll find that it maybe doesn't feel quite as nimble for its size as the slightly larger(?) Omega does, but being inside it for hours on end is a far more satisfying experince on the whole. Going from a nice spec Omega like you currently have to a relatively low spec 75 might leave you wanting for a few toys though. Do some research on the spec levels for the 75 and you'll probably find you need you go up a little from the Classic. I also agree with what's said above regarding the engine choice. You'll probably find the 1.8 petrol wanting compared to the Omega, even moreso if you're a relatively relaxed driver and don't like having to thrash the engine much.
Whatever you do, find a nice example and you won't be sorry