you could also rent the shed separately to one of the tenants for an added fee.As for the lawn, I wouldn't put it right next to the house. 

I am not sure about 403(b) but 401(k) and 457(b) have the option to let you borrow money out of your account up to $50k or half of your vested account (which ever is less).You usually have a 5 years repayment plan (or sometime 10 years for some very narrow loan purposes).If a loan is offered by your custodian, usually you can get a check in a few days and they may charge a small administrative fee.The interest rate is usually a little bit above prime.The interest paid is going back to your retirement account.Now on the negative side:To get the loan out, you will have to sell some of your retirement asset to get liquidity.During the time of the loan, your retirement account will grow slower as there would be less money in it.If you separate from your employer you will have to repay the balance of the loan immediately.

Keep in mind, however, that the IRA(or 401k if you ever get there) is investing in real estate on a standalone basis separate from you. 

Banks are giving me issues because it’s not rented and 3 separate lots on one sale.

Is there any reason I couldn't rent the bottom two units to two separate people?

Depending on billing, you might want to consider separate meters for each section if it is not already set up with multiple meters.On the metal frames, yes colder weather does not affect them as engineered systems are designed for specific locations, to account for snow loads, wind loads, etc.

But I lucked into this home with a permitted garage apartment conversion (separate entry, only shared space is the laundry room).

Try to separate the sales pitch from how they will actually run the business.

However there are lots of items we're allowed to separate out and give shorter lives to even without a cost seg.

Not an expert on this topic but the best success stories I've seen involved situations where the underlying business was done by a separate entity/experts who knew how to properly run the business.