I'd rent a trencher if you will go that deep, but I guess it depends on how much time you have, but you'll also need to fill it back in.
I already had a mini-excavator on property when we were remodeling before moving in. So, I had him dig the ditch, and fill it back in after the line was put in.
Other things to note, the person who put my propane line in added warning tape. Also, if you go 80 ft, you'll likely need a different regulator than most tanks come with to make that distance.
Do shop around for your propane provider. In my area at least, you can pay twice the price of the same product. Also, it's easier than you might think to change providers even when you use their tank. The new company just transfers the propane to the new tank. Usually, they'l wave the rental fee.