I'm in the middle of re-commissioning my S70 T5, and have used the following over the last 2-3 weeks -
Skandix
Parts For Volvo
Autodoc
Caffyns Volvo
FRF Volvo
Amazon
Car Parts In Motion
I've found that you need to shop around to get the best price, balancing that with any additional postage charges due to buying from multiple suppliers. I'm a spreadsheet King
I haven't had any issues with any of the above, barring slow deliveries every now and then.
Re brake discs, I always fit a 302 conversion to my P80s if they still have 280 discs. I've tried various brands but now stick with Mintex or Volvo discs, and genuine Volvo Police spec pads (part number 30645240 without wear sensors, 30645450 with wear sensors, according to my notes). This combo is the best £/performance compromise IMHO, especially for normal street driving.
Obviously for track use you should be looking at something more extreme (and very expensive lol). I've found EBC Green pads to be good too, but not a big enough difference to justify the extra cost over Police spec pads. I've fitted Ferodo DS2500s too which were a good pad, but very dusty and also quite expensive.
Spark plugs - the general consensus is to use genuine Volvo plugs, particularly on T5s.
HTH