You doing it or a shop?
A shop.....I would suggest finding a trailer that is 80% there, adjust as needed.
You doing it......see above....modify as needed......
You doing it from scratch.....see just above....find similar, take lots of pictures, spend a lot of time......

Shop is likely harder hit on wallet....you do it is more time in shop.....


Unrelated analogy, "speed is money, how fastdo you want to go...?"
Also.... "fast, reliable, cheap....pick any 2.....".....