[Q] If I have to find a suitable business and invest BEFORE I apply for I-526, how can I enter the US to do this?
Well, legally speaking, B1 business visa allows you to enter the US and carry out these business and/or investment activities, including but not limited to, finding a suitable business, researching business, meeting and retaining attorneys and CPAs, etc. In short, the preliminary activities can be carried out in B1 business visa.
Even after you file I-526 petition, you can travel back and forth from the U.S., but you might have to explain that you will be returning back to your home country to process immigrant visas.
It's not a good idea to enter the US thinking you can get I-526 petition approved and then file I-485 adjustment application and stay in the US without going out. This is because there is no "concurrent" filing allowed for EB-5 cases.