My rule of thumb (apart from fit the biggest discs/calipers that you can cram into the wheel) is spend the money on the pads. Then get any cheap discs that you can.
These days there are very few discs that are poorly made, so cracking and warping are pretty unusual if the discs are just half decent... although someone will probably turn up a story of some cheapo made-in-China discs they bought that cost £3.50 and warped after two days driving to the station and back.
The braking force comes mostly from the pads. Modern pads are quite aggressive, containing a lot of metal and/or ceramic that a) grips the disc tighter than a Scotsman holds onto a fiver b) creates a lot of brake dust and c) eats the disc. The old days of gentle pads which used asbestos to pamper the disc and only wear the pad, are over.
Be wary of any disc that reckons it's "low dust". If the pad is doing its thing it will be chewing up the pad material and to a greater extent, the disc too so there's going to be dust. Dust is good.
Look at Ferodo DS2500 for pads. This is a fast road pad. You need to jab it a couple of times as soon as you moved off, to get some temperature in it... but once it's rolling and hot, it does the Scotsman bit admirably. It also doesn't wear so much.. so it must be eating the discs more than the discs is wearing the pad.. hence use cheap(er) discs rather that £800 a corner drilled/slotted/carbon-diamond matrix jobs.
Avoid "race-stuff" pads or anything with a colour in the name. They tend to be very hard to resist track-level temperatures.. so they always run cold, don't slow the car and feel like they're made of wood. They also don't make much dust (see above) mainly because they never warm up unless you drive like a berk (this is the roads we're talking about) all the time and once they are warmed up, they cool down again pretty quickly (since they're designed to not overheat). Freddie Ferodo DS2500 thrashes them.
Ralf S.