Do you have any guidelines for the amount of $ expected for the investment required for people applying for the E1 Treaty Investor Visa?

That's the E-2 (the E-1 is the treaty trader visa and not based in investment) and most consulates require an investment of at least $100k regardless of the actual cost to get the company up and running.

Thanks, Peter! What's the most viable visa for European to-be founders looking to move to the US (without affiliation to any institution / accelerator)?

I've been considering (a) going on a M-1 or F-1 master's student visa while working remotely my non-US company or (b) applying for an E1 Treaty Investor Visa. Are there other routes I haven't considered, and are there any unclear watch-outs around these two routes?

For European founders, the O-1, E-2, and E-1 are the most common options, each with its own requirements. What you propose while on an M-1 or F-1 visa really isn't appropriate.