The oe cap and rotor should be retained, but wires, plugs, and coil, in my opinion, should be upgraded if you intend on increasing engine output.

I am personally running Magnecores, and felt quite a difference when I installed them. I have no dyno results, only buttometer feelings, that the engine ran smoother, and seemed to flow through the rpm's smooether. Are they worth the $500 Canadian you have found them for? Not at all!!! Why did I get them then? Because I had a buddy with a wrecked S70T5SE and he sold them to me with about 2-3 months usage for $50 Canadian. As for the plugs, I don't like the stock Bosch platinums. I fine that they don't idle very well, nor do they do well with high rpm downshifting. Their only realy pro is longevity. I highly recommend NGK platinums. I DO have dyno results finding consistent 2-5hp gains with these over the oe Bosch. Will you feel that 2-5hp? Probably not unless your buttometer is as accurate as a Dynojet brand dynanometer. BUT, if trying to get to 300hp+, even if you don't feel it then, you'll feel better about having a better plug. And lastly, the coil should be upgraded. Have you seen the stocker? It's ridiculous. It's mostly underdesigned because Volvo figured the turbo's with their relatively low compression ratios, wouldn't need it. But with electronic upgrades and increased boost, and most importantly advanced timing, and higher cylinder pressure, you'll need the bigger coil. Going at less than $75 Canadian for a really good coil, it's not that big of an investment.

As for things to replace as regular maintenance, look into a Haynes or Chiltons.