How do I store finished builds without wrecking them?

Boxing them up is fine if you do it right, and honestly it beats leaving them out on an open shelf where dust settles into every panel line. Light is the quiet enemy too, since sunlight and even bright room light slowly fade decals and yellow clear parts over the years. So a dark box actually protects them. Use a sturdy cardboard box, wrap each plane in a soft cloth or plain tissue, never newspaper, and leave extra padding around the props. Crumple some tissue into a loose nest around the wings so nothing can shift if the box gets moved. If a prop is relly that fragile, it's common to gently pull it off the shaft, bag it, and store it beside the plane rather than risk snapping it every time you handle the model. Keep the box somewhere with steady temprature, so a closet indoors, not the attic or garage where heat cycles can soften old glue joints.

As for display cases, don't buy new. Big glass cabinets turn up constantly secondhand, estate sales and online listings expecially, and people practically give them away becasue nobody wants to move the things. Worth a look before you spend real money.

On the half built kit, I'd finish it, and not just for the sentimental reason. Masking tape that sits for a couple of years bakes its adhesive onto the canopy and becomes miserable to remove without fogging or scratching the clear part. Leaving it as is actually dooms that canopy. Peeling the tape now and building it is the kinder outcome for the model itself. Put it on the shelf next to his work. He might act like it's weird for a week, but a shelf with both your builds on it tends to be the thing they point at first when friends come over.

That bit about the tape glued onto the clear part never crossed me, glad someone said it. I did the photo thing when my dad's old stuff came to me, took pictures of every plane from a few angles before boxing them, so if anything gets bumped I know how it looked before. Helps with putting props back on too if they come off for safekeeping. Question though, when you pull a prop off like that, does it just lift or do you need to heat the glue a little first? I'm scared of snapping the little shaft thing.

That half built kit is a gift, take it from someone who waited too long. My daughter left a Mustang on the bench when she headed off to college, and I sat on it for two years feeling wierd about touching it. When I finally finished it I kept everything the way she'd done it, didn't fix a single seam or correct her paint, just completed it her way. She got a little misty over it, and now it's the first thing she checks when she visits. Old timer tip for the props: we used to stick a bit of foam from packaging or even a wine cork on the end of a wooden skewer as a handle when lifting fragile builds by the fuselage. Keeps your fingers off the delicate bits.

Update: Taking photos of the models before I put them in storage was a huge help and it was smart. I still have some trouble deciding how to handle the Mustang left on the bench. I don't want to interfere with what she started, but it feels unfinished. If you were me, would you really try to finish it, or just leave it like that?