If the seller is selling both properties as a package deal and not separately pricing them, then, if the two properties are roughly the same value, half your purchase price is allocated to each property for depreciation and capital gains calculations, regardless of how you split up the financing.  

You then have to allocate some of this to land.  

I'd suggest you look at your overall asset allocation and see if it makes more sense to buy the rentals in your own name and put something else in the IRA.

The allocation of sale proceeds will be in accordance with the percentage of the property you allocated to the rental unit when you set up your depreciation schedules.Whether you qualify for the capital gains exclusion on the sale of a primary residence depends upon two tests:  an ownership test and an occupancy test:  Did you own the property at least two years during the fives years prior to the sale?  

Once the in-service date comes, you start depreciating $120k.The trick is that parts of the $120k should be allocated to different items: land, appliances etc. - resulting in different methods of depreciation.

You have to allocate based on business use.

In a S-corp, you have to allocate income/loss items and distribution based on ownership %. 

Examples of deductions, a portion of which may be properly allocable to Gross Investment Income, include investment interest expense, investment advisory and brokerage fees, expenses related to rental and royalty income, tax preparation fees, fiduciary expenses (in the case of an estate or trust) and state and local income taxes.NET INVESTMENT INCOME: Determine the amount of your net investment income by subtracting your investment expenses (other than interest expense) from your investment income.

Your income is like that of a hotel, so you need to determine nightly/weekly rates and allocate a vacancy rate (like hotels do).  

But see section 263A and the regulations under section 263A, which require taxpayers to capitalize the direct and allocable indirect costs of property produced by the taxpayer (for example, property improved by the taxpayer) and property acquired for resale.§ 1.263A-2(a)(1)(i) In general.