A few lenders minimum amount is $50,000 but it's an umbrella loan or portfolio loan.

Assuming it's liability protection (like most of us), you can get an umbrella insurance policy and also move the property to an LLC - double coverage. 

I still haven't been convinced by anyone (through multiple discussions) that an LLC provides better protection than a personal liability umbrella policy.

You also need duck feathers and an umbrella; you need to let things like overdoses, police visits, prostitution, underage drinking, breakage, theft, accidents, lawsuits and other common things like that slide off your back. 

That's in place both in the areas of the property and personally (property, umbrella, etc).

I have a $1mm umbrella policy in addition to individually insuring any properties that we have in progress.

Pair that with a solid umbrella insurance policy.

The brokerage account option is great for those that want to be able to do all under one umbrella (i.e., invest in real estate, stocks and mutual funds and process the solo 401k loan).

Is there any way to work things so that we could secure 30 year mortgages on these properties and keep them under our S Corp umbrella?

Instead of an LLC if it is for asset protection I would suggest leaving it in your name so you don't risk the mortgage being called due and instead go with an umbrella policy to provide additional protection beyond what your property insurance covers.