With all due respect:
The Opel Omega MV6, built in their old Russelsheim plant (now torn down), was a fine and well respected automobile in Germany. The car was, and remains today, Autobahn ready at max speed of roughly 140 mph with the correct tires. This is true also for the Catera-badged versions which can be picked up today in the USA for a song. I think some owners might even pay you to tow their Cateras away! 
Fact is the MV6 was, and it remains, a German automobile. Simply because it was sold as a Cadillac changed nothing whatsoever. There are Cadillac mechanics and there are German automobile mechanics and they are not at all the same thing. Here is what happened:
Back circa 1995-1996 the geniuses at Cadillac got the bright idea to rebadge MV6s as Cateras and sell them in the USA. To make matters worse, those same nincompoops decided to charge stateside Caddy dealers $50,000 each for the privilege of selling the Catera. Many dealers did not buy in which meant Cadillac refused to train any of their mechanics on the German Catera. But Catera owners were never told which Cadillac dealers had Catera trained mechanics and which did NOT!
The Catera was a German car requiring skilled German maintenance by the kind of mechanics found there . . . . but not here. And that includes especially preventative, routine, maintenance which is not at all routine for American iron.
The result of all this was Catera acquiring perhaps the worst reputation of any (supposedly) American car ever marketed. It really gave Cadillac a black eye and deservedly so. The Cadillac brass messed Catera up big time.
The bottom line, though, is as follows:
If you can locate a low mileage Catera in nice shape today, and if you can maintain the car as would have been the case (and was the case for the MV6) in Germany, you will have a 140 mph Autobahn cruiser for VERY small money.
This is a situation I have watched for many years. I was a prominent poster, for a time years ago, on the Catera discussion page. I have tried to locate a German car mechanic so that I could purchase a Catera for myself. No luck. Here where I live such mechanics are incredibly scarce. But were I able to find such a mechanic I would buy a Catera in a heartbeat because they are luxurious automobiles and you can buy them for a song.
The only caveat, as before, is that prior to taking the car up to its 140 mph design speed you MUST fit tires which can handle such speed. And when sold here new, the Catera was not fitted with those tires.
Oh, one other thing. You might also have to disable the Catera’s governor. I think the factory set that to just 120 mph. But the MV6 never even had a governor . . . . and it’s the same car.
ETA
The other option is to school yourself and then do your own Catera maintenance. This is what many current-day owners do. And they love their Cateras. But you really have to know what you are doing. The Catera engine, for example, is an interference engine with a “rubber” (as opposed to steel) timing belt. It’s a recipe for trouble especially for people living in cold climates who park their cars outdoors. At one time Cadillac’s suggested maintenance interval on that belt was 80,000 miles!! My own recommended interval, depending on climate, was 40,000 miles. Because:
If that belt goes you lose the entire engine. And that actually has happened to a great many Catera owners. It’s no wonder the car has such a poor reputation!
But when those Catera engines are running properly they are high revving things of absolute beauty. It’s a great engine but only when properly maintained as the Germans originally intended.
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