I would definitely do a large display only cake made with faux tiers
and then kitchen cakes for cutting
you could do 20 three layer half sheet cakes, that would feed about 75 guests per cake if you cut industry standard of 1"x 2"
then there is the issue of storing 20 cakes. You will likely need a commercial kitchen space or a large deep freezer at the least.
with a large deep freezer you can bake and freeze all the layers ahead of time and then defrost, fill, stack and ice the day of.
Van rental is a must, you should be able to rent a utility van for about 90$ plus gas and maybe mileage. you'll have to work out your best option with the rental place, of course
you can do it, just pleassseee don't undercut yourself for payment. This is a huge undertaking and family or not you deserve to be compensated properly
best of luck :)