Bit of both, upgrade and replacement. The XF stuff is 20 years newer and while not that different it is better, the aftermerket bigger front sway bar, springs etc will tighen it no end, Kings springs are about $100 a pair, set of KYB gas adjust shocks will be about $130 a pair. replacement ball joints etc are just falcon full set uppers and lower around $100 set for good quality ones, spring pivots similar. $100 worth of other crap like bushes, camber pins etc, sway bars around $160 each. It's important not to get too carried away with really stiff front end set up on these cars or the flex like hell and a well put together one has pretty tight gaurd to bonnet gaps. I've seen them rub paint off the bonnet edges when driven hard with stiff front ends. I've got the LHD power steer from my car if you want it. Don't know what it's like probably needs freshen, but my car was not very knocked around just well driven and faded, never been painted since new apart from some touch ups. Never damaged in the front so the steer would be ok I reckon. You can get really carried away buying trick aftermarket suspensions for these things, but I don't see the need, I don't really think they need to drive like a BF falcon, as long as they corner ok and stop ok, they are fine to drive, and again you can buy generic ford bits anywhere in Australia, you will never be stuck. That's my view away, They should drive a bit like an old car albeit a good old car, after all that's kind of what it's about.
Make sure Steve puts 2 torque boxes on it to firm it up while he's there. It's looking good what colour are you painting it underneath? body colour, black, red oxide?