Hi Mike,
Good to see that the issues you've found haven't put you off, people give up on cars for less! If, like me you see the S-Type as a long term project and a 'keeper', it's well worth making these repairs and keeping her on the road.
I do most of my own 'spannering', and did change my drop links, which was a doddle and inexpensive parts too. But mine failed MOT on a wishbone last year and my MOT mechanic offered to change both sides for £100, including swapping some tyres around, so I went for that rather than DIY. Slightly p****d off I did as they overtightened the wheel nuts when replacing the rear wheels and I still have a broken wheelnut I need to remove. At a guess, a couple of hours a side for a DIY'er?
I can recommend using Berkshire Jag Spares for parts, knowledgeable and good quality. I paid £136+VAT per side for the wishbones with OEM bushes, the droplink was about £15 I recall. Not sure if yours is the same set up as my X200 (2001), but hope this helps anyway.
Good luck, Russ