Current lens is 18-50mm
nx mount lens is WAAAAAAY expensive
if i buy an adapter and a cheaper second hand lens what functionality will i lose?
Will i have to set aperture manually? what about auto focus – will that work? Auto focus i am not that bothered about.
Mainly looking at lunar pics

Thats the trouble with Samsung these days, when they first got into DSLRs they were basically rebadged Pentaxes. This meant that you basically had access to pretty much any lens with a Pentax K mount. Because they are using their own mount the choice of lenses is pretty limited. I don’t think any third party lens makers such as Tamron or Sigma make lenses in an NX mount.
You may find this article useful however.

What about 2x tele converters? any good?

Teleconverters in general aren’t that great. They’re useful for getting the shot, but they are generally quite slow, image quality and exposure can be variable and not all functions may work properly.
I’ve got one, but it was quite cheap and I got it to play about with.
The only advice I can give is to try one and see what you think. Some are better than others. As with most things in life, you get what you pay for!

Mainly looking at lunar pics

You could try attaching the camera to a telescope. You can get various sorts of adapters (look up ‘digiscoping’).
Probably the cheapest way to get a lot of magnification. Though you will have to manually focus the telescope and the camera.

I’ve got a nx300 and use an ancient Russian lens. I set it to apeture priority and manually focus. Its dead easy to adjust exposure etc. I use a fotodiox adaptor and its good quality.

Which lens system were you thinking of?

that sounds like a plan – any more details?