Due to space it would have to be sit down. Not fussed by budget.
Can you wear headphones with these? My DT990s for example?
Well, with the Quest 3 space isn't an issue if you want to play native Quest 3 games (or even PCVR games if you set it up correctly) You can use it anywhere you have space, so you can play games like Batman, Asgard's Wrath 2, Assassins Creed in the garage or sitting room or Kitchen etc. You can play PCVR games anywhere in the house where you get a good signal from your router, as long as that router is hardwired back to the computer.
AS I said there are other options. The only two I would really suggest for you are the Pimax Crystal Light and the PSVR 2. They both have their advantages and disadvantages. For your use case, sit down sims only, the Crystal Light is by far the best option. On paper least. While the PSVR 2 still has fresnel lenses, it has OLED screens which are really nice for games like Elite.
The Pimax Crystal Light is the the best PCVR headset of the 3 suggested. However, Pimax does have quality control issues and their headsets do require a bit more faff to setup than the others listed. The quality control issues seem to lessen over the lifetime of the headsets though. So chances are you will get a good one but it's just something to be aware of.
The PSVR 2 is another great headset on paper. But it uses fresnel lenses and has a small sweet spot. The OLED screens are great but there is mura. It might not be something you notice and might not even bother you in the slightest if you do. You will have to get an adapter for your PC to use.
Quest 3. For PCVR use it connects over Wireless or USB, which means compression. Compression can lead to artifacts on the screen. Again, these might not even be noticeable to you and might not be an issue for you even if you do notice them. You can minimise these by changing settings. Also the stock strap isn't very comfortable(for most people) So you will have to get another strap. You will also have to buy a decent quality USB cable. You might want to get other quality of life upgrades, like a facial interface and prescription lenses if you wear glasses. You can wear your DT 990's with most of the headstraps.
You see the problem with recommending headsets? A lot of it depends on the person who is going to be using it. Their tolerance for things that might reduce image quality. How the headset fits on them etc. etc. Moreso than any other device, there is a lot of personal preference involved.
Is there your first VR headset? Or even the first time you have tried VR? If you have never tried a VR headset before, then it makes it a lot easier. Any of the headsets above will absolutely blow you away. Probably the easiest headset to get started off in is the Quest 3. But none of them are anything difficult to get going.