First job for you must be to replace those cracked side-repeater (indicator) lenses and any other lenses (headlights, etc) because that's an instant MOT failure.
Here's what I'd do, and in the order I'd do it in:
1) Replace the lenses (as above)
2) Give the interior a good thorough cleaning (no offence to your sister, but why do so many women let their car interiors get so messy?) and ditch the steering wheel cover. Replace the ashtray as well if the green paint won't come off.
3) Floor mats for the interior (the actual carpets don't look to be in bad condition, mats will keep 'em that way)
4) Replace the missing passenger airbag cover
5) Replacement rear bumper
6) Paintwork repair on all the body panels (stop rust from setting in)
7) Find and fit a nice cheap leather interior from a 406 D8 "Executive" (including a leather steering wheel)

Find and fit a pair of front-facing fog lamps (should technically be quite easy to install on a D8 like yours)
The wheels you've found are nice - one looks like it could really use a refurb, but I'd also replace the centre caps with a set of Peugeot ones (measure the size and check the fitment of the ones that came with the wheels and find a set on eBay that has the same size and looks like it has the same fitment method). Will certainly lift the car's appearance.
Otherwise it looks to be a good solid car; I can't see any signs of rust on your photos, and if the engine runs well I don't see any reason why you couldn't get another 100,000 miles out of this motor - just keep an eye on the cambelt change intervals.
Enjoy the motor, look forward to seeing it again once you've done your bodywork fixes!