IMO it would depend on what you want to make of the car.
You say bodywork is scruffy & interior is beyond cleaning, so do you just want a reliable workhorse where appearance does not matter.If that is the case, then doing all the mechanical work is the way to go, you know that all the major issues are sorted.But seems like a few things need to be done ,so gonna cost a few quid
On the other hand , there are lots of omegas out there for sale and some good buys too.My 3.0 FL elite on a W plate was £850,searched 6 weeks to find it though, was looking for a clean well maintained 3 litre model, fully loaded with CID etc,& with a recent cambelt change. was up for £1100 . . . bit of Essex Boy banter got it down to £850

You have to weigh up what your outlay on the bits that need doing on your exisiting miggy are going to come to against trying for a "new" one.
You would get £250 weighing the old one in at Rochford scrap at the moment, don't know what the breakers on here would be willing to pay though.
To sum it all up Adrian, being honest & without wanting to offend you

, having seen your car and the cosmetic condition, I would go for a" newy", but take your time in searching one out.
Hope this helps your decision, regards, Al, PS. kettles always on if you wanna pop round
