getting into the streaming world.
And my question is :do I invest in a Mac book or a streamer ? Or should I approach streaming  in a differenent way?
My budget is around 1800 dollars.
Thank you for your help.
Depends on what you want to stream. Local files? Streaming services? The computer used for other things? What are you connecting this to, a system with speakers or a headphone system? 
computers are inherently "noisy" and not built specifically to handle just music. IMO, a streamer is a better choice with a dac you like. I use a one box solution, a Lumin T2. Previously I ran a MacMini and did everything possible to make it "sound good" The T2 was so far superior it's not worth a comparison.
I think using a Mac is overkill. Roon, BlueOS, and Volumio are things you should look into.
computers are inherently "noisy" and not built specifically to handle just music
Because of that I decided to break connection between computer and audio system by WiFi.  That way nothing on computer side matters (playback program, computer speed, amount of memory etc).  What is being transferred over WiFi is just a data in packets.  I have to worry only about receiving end.  Airport Express I use has decent jitter on optical output (280ps), but it can be improved.  I've read somewhere that audibility of jitter starts around 50ps.  Because of that I selected DAC with built-in re-clocker (Benchmark DAC3) that suppresses jitter.  This solution also allows me to use computer for other tasks during playback.  Dedicated streamer is perhaps a simpler solution.

For your budget you could also explore a NAS and a streamer. I have a WD server with a public music folder and Bluesound Node 2i. This provides me access to internet radio, streaming services and the CDs I ripped/downloads. 
The value of my music collection is using only 1 streaming service. For example, I owned multiple CDs from Iron Maiden, Garth Brooks, Kid Rock  and Metallica. Tidal doesn’t have the some of the tracks. 
My preamp has a quality you may find a separate DAC a worthwhile investment. 
Thank you for all the answers.
I will start learning about/looking for  a streamer/dac combination.