Hold on for a second.....are you building this motor yourself? If you are having a shop do it, they should be able to order everything that you need for you (once the motor is torn down). If you are building it yourself, it sounds like you have a lot to learn in a very short time and you probably aren't ready for this (yet). But again, assuming you are having someone else build the motor for you, I would advise against just up and buying some pistons. Once the motor is torn down, 1) you will need to decide if it needs machining (bores, valves, etc), 2) given the nature of your project, you will need to determine what kind of pistons to use, what physical properties (like land thickness, squish, effective compression, etc) you want. All of those things will need to be actually looked at, and could very well affect the dimensions of your pistons.....dimensions that your motor may or may not share with other m103's. Also, spending $1500 on a set of custom pistons right now, that you may not be able to use later, with little chance of selling, is probably not the best thing to do. I know it seems as if buying the pistons (or whatever else) will make your project happen sooner, but sometimes this is a waiting game. Good luck

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1999 C43 AMG ~~~ 744 on black, mostly stock
1991 190e 2.6 ~~~ m117 5.6 project car