Let's say my Thai friend buys a parcel of vacant land (with my $$).  Then i get a usufruct for that land. 

Am i permitted to do all the work to build a simple shed/house or must i hire Thai folks for all physical labor?

 

What if i bought and erected a big tent on the land, slapped together a few outbuildings and called that 'home'.  Is that permitted?

Depends, in Bangkok, probably NO, out in the boonies of Issan probably, YES

 

The authorities normally won't give you any problems working on your own house so long as you have a building permit  Which may require you to submit "plans" which may or may not be architectural drawings (blue prints) but are never checked during or after construction and can be renewed annually for a small fee 

 

But your mileage may vary depending upon your location 

1 hour ago, Langsuan Man said:

The authorities normally won't give you any problems working on your own house so long as you have a building permit  Which may require you to submit "plans" which may or may not be architectural drawings

If you are far enough out from a town you don’t need anything, no building permit, no plans to submit. 
 

We didn’t need to anything.