[Seggy]

In order to rent and fly without a CFI in instrument conditions, one must get checked out with a CFI before hand.

That CFI is to go over decision making, with the potential renter, weather, and other factors that are important in making go/no-go choices.

Then the student is limited to flying ONLY when the visibility is greater than 3 miles and 1000 foot overcast skies primarily along the route of flight and at the destination airport.

With an instrument student and CFII in the airplane, I would bring it down to greater than 2 miles visibility and 500 foot overcast skies.

You must have three airports with instrument approaches, two of which must have an ILS, that you can use as alternates within an hours flight time from you at all times.

Winds greater than 20 knots are prohibited and flight into known icing conditions are prohibited as well.

That is what I would start with and go from there.