These questions were posted in the bracket group buy thread, but I chose to answer them here as they seemed more relevant here.
WAIT.can't I use the oem euro floating rotor with this caliper? Or do I have to buy one with the new caliper? What is the best place to purchase the 135i calipers, I don't like the yellow.
Purchase 135 calipers
Well if you do not like the yellow the best place to purchase the 135 grey calipers is from a 135 owner who is getting rid of their set up. Keep on eye out on bimmerforums, 1 addicts and bimmerfest maybe even post a wtb ad on one or all of those forums. You could perhaps try ebay, craigslist and/or your local auto salvage yard or maybe a larger place like Bavarian Auto Recycling. Of course you are going to be getting something used so the condition of the calipers may vary.
The other way to purchase the 135 calipers is from a
discount BMW dealer like getbmwparts or your fav BMW parts supplier, but just understand it is going to be more expensive to buy the 135 parts separately than buying the PBK kit. Example prices from getbmw parts for two 135 calipers $1,158($579 each) [I am not sure if the bracing spring, guide pins and bleeder valves come already on the caliper or if you have to buy those extra], two 135 plain rotors $271.40($135.70 each) or performance drilled/ slotted rotors $396 ($198each) or ECS who has them for $329, pads would be $164.46 (not sure if that is 2 or 4 at getbmwparts) or $177 from ECS although you can probably find something aftermarket for cheaper or perhaps just a compound you like better. So for a new 135 setup you are looking at $1,429.40 ($1,554 for drilled/slotted rotors) not including pads versus the PBK is $1,348.30 which includes complete calipers, rotors, pads and wear sensors. Depending on your financial situation the cost may or may not be an issue. Perhaps if you shop around you can do better on prices. You probably already know this just don't go to your local BMW dealer unless you have money to burn as the local dealer price will be significantly higher MSRP for the calipers is $724 and many dealer charge more than MSRP for BMW parts.
135 font caliper parts for reference
Front wheel brake-Brake pad sensor BMW 1' E82, 135i (N54)
Floating Rotors
If by oem euro floating rotor you mean the e39 M5 floating rotor then the answer is no it will not work with these brackets. These brackets are designed to hold the caliper in a spot specific to the offset of the 135/PBK rotor. I do not think the m5 rotor shares than offset, but even if they did the caliper would not physically fit over the rotor as the M5 rotor is too thick to fit bwteen the brake pads of the 135/PBK caliper.
The oem 135 rotors are plain rotors (see post 204) and the rotors that come with the PBK kit are drilled and slotted rotors (see post 205). The rotor hats do appear to be attached with rivets or something versus a cast single piece of metal for the rotor (applies to plan and slotted/drilled), but it is not the same design as the M5 floating rotor pin attachment setup. There are aftermarket 2 piece lightweight rotors made by RacingBrake, but they are expensive see post #84.