I want to test a SaaS product but I don't want to set up a company yet. I just need a temporary method to handle payments & legal stuff until I validate the product.
You should probably check in to sales tax in the US and Europe that you might be responsible for. Checkout TaxJar. In the beginning this is less critical as most have minimums that apply, but worth checking in to.
Once you have revenue you can form an LLC or other entity.