What is your experience? What machines do you have? If you already have a decent set of machines and tooling and can use them, then money is your only restriction. If you are starting out, you have two choices, bar stock or castings. If you are close to metal suppliers, then there are numerous bar stock plans out there for free. Then your choice is aluminum, brass or steel.
If you pick castings, then there are two basic choices, PM Research or Stuart Models. Simply pick one in your price range and Google it. Both offer replacement parts if you make a mistake. There are so many videos of the starting engines, that you almost don't have to look at the plans, just follow the videos. Watch Keith Appleton videos. He recommends the S50 (I think) because the parts are larger, but is twice the cost of smaller ones. Simply walk down the list of questions and you should be very close to a decision. Your biggest choices for everything is the UK. You will find most plans, parts and models originate there. They have some really fantastic models for around $3-4K and will take you a year or more to build. You can dream about them when working on a Stuart 10V.. There are tons of videos showing the different model engines shows here in USA or UK, so you can probably see what you choose in operation.
Me, my first engine is a steam donkey from plans. Already started collecting the parts. Figuring on a year or so to do.