S40: Disc and pad renewal

[P156KWJ]

As I'm still new to maintaining cars (despite having worked on buses for 7 years), was wondering if anyone could give me some pointers regarding brake renewal on my S40.

I think the front discs are warped as there's the telltale sign of some steering wheel shake when slowing down from a high speed, and at a very low speed with the pedal steady, it feels as though when the wheels are still rotating, the pads grab slightly more on some parts of the disc than others. The rears are quite obviously getting ready as there's quite a prominent lip formed on the discs. Anyway...

Euro Car Parts and GSF are not exactly helpful with various options for my reg number although the rear axle seems to be easy enough for parts. The front axle there's various discs, although I believe it's the 281mm variant rather than the 256mm (short of waiting until I've stripped it all down and measuring the old disc). However, there's two variants of pads that are different sizes, and I'm not sure what exactly is compatible.

Volvo would like a fair amount more money than Euro, is it worth the extra just to avoid any potential hassle and just get the OE parts?

Any extra pointers or issues I may come across? Got plenty of tools, but is there anything special I may need?

Thanks in advance!