I am making my 1st cake to feed 200. (5 tiered round).
I have never delivered a cake where I have to set it up on site. How much should I put together at home? Bottom 3 tiers? Does anyone have any tips for delivering and setting up on site?

Pre stacking the bottom 3 tiers is going to be REALLY heavy. I did a 5 tier cake last December...my biggest. I pre stacked the top 3 and even that was too much. If I were to do it again I'd transport in 4 pieces...top 2 already stacked and the rest to be stacked on site. make sure you bring everything you need to set up, including enough frosting for your borders.

One of the things that I hadn't predicted when making mine was just how much SPACE that much cake takes to work on. Do you have space to be working on 5 tiers of cake? If not, figure that out ahead of time. Borrow folding tables if necessary.

Allow yourself plenty of time! Next time, I would make it my goal to have the cake ready for transport before I go to bed the day before it's due. Mine was for a Friday wedding. The flowers were done well in advance. I baked and froze a week ahead (FYI, it took 11 hours over the course of 2 days to bake...you do have somewhere to freeze all this cake, right?); I made all my BC on Wednesday night; on Thursday night I trimmed, filled, and crumbcoated my cakes. I had taken Friday off work; all that was left was to cover in fondant, prep for stacking and decorate. NOT ENOUGH TIME! I should have backed everything up a day.

Sorry, this is probably tmi but I always hope to alleviate someone's stress with what I learned

Good luck! I'm sure your cake will be great!