Welcome aboard Steve.
Model Engineering is a broad church, and I'm not into modelling as such myself. However, I can recommend the PottyMill as a starter project. I made one as No castings needed, 3 different metals, learning to read the drawings, planning the work, and a variety of build challenges hard enough to make the beginner think, but not difficult enough to be discouraging. And it's a real buzz when an engine you built yourself works, even if it is a bit rough!
I used the same engine to learn 3D-CAD Assembly, Joints and animation. This is part-built in Fusion360:
Next trainer engine for me was Jan Ridders' Coffee-cup Stirling:
This is a much more delicate engine because it works on the heat from a mugful of hot water. There's not much energy available, pressures are low, and friction and leaks are highly undesirable. The Meccano motor pictured is 'running in' the engine.
Next engine was going to be JasonB's Flame Licker (build thread here), but after collecting most of the materials, I got diverted. (Many others – Jason knocks out engines faster than I type.)
Please share your successes and failures, and we love photos!
Dave