You can finetune whisper, stable diffusion, and LLM up to about 15B parameters with 24GB VRAM.
Which leads you to what hardware to get. Best bang for the $ right now is definitely a used 3090 at ~$700. If you want more than 24GB vram just rent the hardware as it will be cheaper.
If you're not willing to drop $700 don't buy anything just rent. I have had decent luck with vast.ai
No clue but if you want learn/finetuned ML use a Linux box otherwise you will spend all your time fighting your machine. If you just want to run models Mac might work.
Which leads you to what hardware to get. Best bang for the $ right now is definitely a used 3090 at ~$700. If you want more than 24GB vram just rent the hardware as it will be cheaper.
If you're not willing to drop $700 don't buy anything just rent. I have had decent luck with vast.ai