A late 90s 7 series is going to have continual electrical issues, always expensive. Don't ask how I know
. That's why they are peanuts to buy, often pre LPG'd by previous owners.
And 4.4/4.8 beemer V8 with a few years on it is going to lose water and oil faster than your Omega does, although usually externally.
For the Jag, you need to ideally go for the Ford derived 5.0l, as water pump and a coolant hose aside, is rock solid. That puts you post 2009 XF X250 (earlier is 4.2, X260 didn't get the V8) or XJ X351 (90% certain the X358 only got the 4.2). Because this engine is so much better, they command strong prices. But do offer 400 - 575bhp factory depending on induction and tune, and similar torque to the TDV6 3l diesel lump.