Couple of ideas call out to me here. Pallet racking is affordable for heavy storage, but it isn't the easiest to move around without disassembly. But you could park a full motorcycle on any shelf without any fear of support assuming you can get it up there in the first place.
Wire racks or "metro" racks are nice in that they are configurable, and six legs can yield two horizontal sections of shelving by sharing the middle legs. A pair of shelving unit kits can be spaced out to give you four units. The they have wire shelves and those things are annoying as hell for anything that can get caught in the spaces. Tubs help a lot there, but you'll still have things you will store that will catch on them. Putting a piece of hardboard on top of the shelf will help greatly.
For a shop like yours, I think I'd be tempted to go with the wire racks and put wheels on them so you can pull them out as complete units and move them to where you need them during work. I'm a huge fan of wheels and being able to reconfigure the space as needed.
I would recommend if you have stationary shelves that you leave the bottom open to the floor for the first 30" or so. That will give you space for engines, heavier tools or other awkward shaped items (on wheeled dollies). Also tether the top to the walls with earth quake straps or similar.