Technically they all can because it relies on software, it's just that NVLink will reduce the performance penalty going between GPUs. There is no free lunch here so you damn well better know what you're doing to not get stung like this guy by speculative bullshit pushed by people who never actually had to make it work.

With that out of the way, it doesn't get any better than ex-mining 3090s that start at ~$600. Don't bother with anything older because if your problem requires model parallelisation, than your time and effort is probably worth more than the pittance you save in trying to get some old 2080Tis or 2070 Supers to keep up.