When it comes to buying parts for your Core-XY build there are many things to consider. Are you building a open source design or are you designing your own? Are you going to use smooth rods or linear rails? Do you want to build a printer on a budget or are you going to build something that performs and is a machine that you can show your friends. Whether your building a pre existing open source design or a diy home build the first question you'll have to ask yourself is do you want to use linear rails, smooth rods or delrin wheel carriages. Most of the open source corexy designs on Thingiverse use either smooth round bar with linear bearings or carriages with wheels. Although you may still find some really good designs that use linear rails. Lets look some of the most common builds off of Thingiverse: