Stand in front of the display wall and ask yourself one question before anything else: what is the room already doing color wise, because that's where most trees go wrong. A heavy red and gold scheme sits badly against terracotta walls or a rust colored sofa, and metallic silver baubles can look cheap next to warm wood tones that lean more amber than honey. If your space already has a lot going on, patterned rugs, bold art, a feature wall, go quieter on the tree rather than louder. Two strong statements in one room read as clutter, not festivity. I learned this hanging a full set of jewel toned ornaments in a room with a deep green accent wall and it looked like the tree was trying to disappear into the plaster.
Before you buy anything, measure the space where the tree or the mantel display will actually sit, not the space you wish you had. A six foot tree needs roughly four feet of floor diameter once the branches are out, plus another foot of clearance so you're not brushing past it every time you walk to the kitchen. For wreaths, measure your door or window width and come down a few inches on each side, a wreath that's too large for its frame looks like it's trying to escape it. The common mistake is buying ornaments at the size they look like in the box, which is always smaller once they're thirty feet away on a branch. Go bigger than feels natural for anything you want to actually register from across the room, and buy a few oversized pieces rather than a lot of small ones to fill space.
The difference between a good set and a disappointing one shows up fast once you're actually hanging things. Cheap baubles have thin plastic shells, visible seams, and hooks that bend the first time you touch them. Good ones have weight to them, glass or thick resin, a properly finished cap, and loops that don't need replacing the day you unbox them. LED strings are the same story, cheap wire goes brittle and the lights fail in sections after one winter in a box; better ones have rubber coated wire and replaceable bulbs. None of this matters if you're decorating for one party and tossing it after, buy cheap and don't look back. But if you want it coming out of storage for the next decade, spend on the pieces that touch your hands most, the ornaments and the lights, and save on the filler.
I'd start in the living room, where everyone actually sits long enough to notice it.