Somewhat late, because you've already sorted your new alloys, going by other threads, but the other letters on the tyres are usually other codes for the sizes.
There are currently 4 or 5 different ways in which tyre sizes are expressed, these are mainly country oriented.
The exception is the 87Y part, this is with reference to the compound used to make the tyre. The codes are based on the maximum sustained speed of your vehicle. It may be that in practice you would never drive for long periods at over 80mph, for example, but to be safe people usually use the maximum speed of the vehicle. 87Y would actually be suitable for a car doing a sustained speed of up to 190 mph, so I think you should be fairly safe. Anything ending V,W,Y or Z should be good enough. In theory everyone should be safe with any tyre, because the minimum rating is N, which is safe for sustained speeds of 88mph.
There are a couple of exceptions to the normal lettering scheme, they are VR and ZR. Usually expressed as VR17 or ZR17, with the 17 part referring again to wheel diameter. The speed ratings for these tyres are expressed as - vehecles with a speed capability of over....
In your case you have ZR17 87Y. The ZR17 is means capable of over 150mph, the 87Y means not exceeding 190mph. The two codes can be used in conjuntion with each other or individually.
Maybe helpful for future use.