Well, first the easy things:
Oil pressure sensor can be easily exchanged. Maybe you buy it from www.autohausaz.com
I usually buy from them as they have a good service and decent prices. But compare with others too. Do a search here for BMW online vendors list (see top of this page the search).
Power steering fluid low: as you have a 750 iL, this one has LAD. And the LAD system and the power steering have the same fluid system, and they use Pentosin CHF 7.1 in your case (see sticker on top of it). That reservoir has NO dipstick to check the level. How did you check it? Which procedure did you make before checking level?
If you have a U.S. specification car, you can make a socalled stomp test, which will give you some numbers on possible faults. Pls visit my website below and see the procedure for stomp test and let us know the numbers you got.
Easier it will be, if you buy a PEAKE fault finder, www.peakeresearch.com
You also say check engine light comes on. What faults are stored in the check engine and are displayed? You can see the results on the CC. If not , see the owners manual how to get them. If you do not have an owners manual, see my website below and download it from there.
Let's see what you find out and then tell us, we can help you.