I'm going to be brutally honest here, so those of a nervous disposition please look away now...
Is the car standard as it left the factory & is this engine the original engine for this car ??
The fact that you are asking these questions makes me think that you ought not to be buying it.
It's OK to buy something you don't know much about - and after all, buying something you don't know much about at least ensures that you
will learn more about it - but you're putting yourself in a pretty poor starting position if you haven't done at least enough homework to answer those two particular questions. They are - being honest - pretty basic stuff that I'd expect somebody who was thinking of buying a C10-series Skyline to have swotted up on a bit. This kind of data is in all the books on the C10.
Sorry if that sounds harsh, but I think it's true.
In answer to your questions:
Yes, it's pretty much stock. Only the wheels and tyres and a few other small bits and pieces are non-stock as far as I can tell from the limited pics.
Yes, that's almost certainly the L20A engine that it left the factory with. Single carb would indicate that it's a 'GT' model as opposed to the twin carbed 'GT-X' model. The Mitsuba electric fuel pump in the engine bay is a non-stock addition, and was probably added in an attempt to help with fuel percolation issue when the aircon was being used.
The car will soon be in need of a FULL tear down and body restoration. If you don't want to get involved in that, then you would be best to avoid it.