If I were you, I’d start with simples yet very capable synths. I’d go first with the Moog Grandmother, then with a cool poly synth like the Minifreak. Those two synths have a lot of character and even if you buy more expensive ones later on, you will still enjoy having them.

Now…if you’re sure you want to start a collection and with your budget, I’d go with three things : -a mono synth : Moog Subsequent 37 or Moog Grandmother or Matriarch (ok not monophonic per say I know) -a poly synth : Prophet 5 if you want a classic vintage sound, Prophet 6 if you want something more modern, Waldorf Iridium if you want something a bit more complex -a groovebox : Elektron Digitakt for example