Some random things:
Your budget is $10,000, 10x the amount Sandbox had. It's a bit misleading though because the base price for many of the objects seem to be twice that of Halo 3 so 5x Sandbox is a better estimate. Then you see "buldings" which all seem to cost $150 which is the equivalent of 10 warthogs in Halo 3. That original variant they showed off, pictured below, seems to take up 2/3 of the budget. It's a decent sized map with many pieces but for some reason I expected the budget to be much greater. People wanting detailed remakes of large maps like Headlong may have to cut some corners to stay within the budget. The linking of all objects within a certain category puzzles me.
When floating around you have two options:
1) Tools
2) Place Object
Place object leads you here:
Structure goes to:
Building blocks:
While holding a block these are your options:
Buildings list:
"Special tools"