Second hand ebay for components to build = expensive route. Think finding a donor bike, with a group you like, then move it en mase to your Rossin. It also eliminates most of any compatibility issues as you know the group works well together on one bike (exception for bb, seat post and hs, those are frame and COO dependent). One problem with a donor bike is if you are going to have a shop do all the work, it can run up some significant labor charges.

As you plan to have a shop do the work, talk to them about what groups to use and they may have something in hand that works.

Since you are desiring a specific model and size, be prepared to pay a premium, and ship from wherever. Also be open to a complete bike, even if the parts are not what you want, the sale of those surplus parts can help offset part of the cost. I try to avoid locking into a specific model. In the one case I did, I paid a premium for sure, and bought a complete bike with components I did not want. I also had it shipped a fair distance, and the seller did a mediocre job packing it.