what should I build?

A million kg to orbit. Anything that is exactly 1 million kg to be launched traditionally (rockets). If using fuel as weight, you cannot drain fuel from the weight. Bonus points if it crashes into a lunar base

You should build like a kind of SSTO plane

EDIT: I was working on one but it was just sub-orbital (oof)

@DPSAircraftManufacturer i didnt know that, interesting

Here you go

A rocket

ESA is working on some interesting reusable space vehicles that you could try your hand at building:

Space Rider
Airbus' Adeline