Understanding the specifics of your opportunity are necessary to provide a sufficiently detailed answer. Based on the information you have provided it sounds like the seller is looking for investment as opposed to a loan, so it may not be possible to offer a shareholder loan as a percentage.

As a shareholder in a private entity, it is important to have a very clear understanding of the rules that governs the corporation itself. Unlike publicly traded firms, most shareholders of private firms are active in their investment.

Will a 30% share grant you the option to be involved in business decisions? If not you may find you have challenges receiving any return on investment as management will have considerable latitude to determine whether there is any net income to pay out as dividends.

Does the 70% shareholder work for the company as a full time employee? How is their compensation set? Will you have any oversight in this regard?

I would strongly recommend you approach this deal by seeking a shareholder agreement that is similar to a partnership agreement where the major decisions are made through a process that involves you, while the day to day is managed by the active investor.

I would also ensure that your ROI is sufficient that your capital investment is replaced fairly quickly as it does not sound like your investment will have much, if any liquidity to it.

If there are more details of this you can share, I'd be happy to provide greater specifics.