There are too many factors to consider to just provide base rates for what you're asking. 

Is there a way to passively track mortgage rates on 5+ residential apartment buildings?   

A line of credit is adjustable rate with interest only payments that will have a "draw" period usually of 3-5 years for investment properties (usually 10 years for primary home) then will amortize over the remaining term after the draw period.

Do you look up competitor rates?

Of course this all depends on YOU setting the base rate and all the knobs and triggers most appropriately.

Charge the tenants a higher rent rate and include utilities with their rent.

The rates vary throughout the State because of the different millage rates and tax districts, but in Richland County for instance, the taxes essentially triple once it goes from legal resident to non legal residence. 

GC's get paid off of the work, not a flat rate... but in your experience, would you say that window of rate is generally 10%-20% project depending, or more towards 15%-30%?

Let's break it down with precision:The "Pain" (At First Glance):Your $2.8M sale splits out as:Building (§1250): $2.3MPersonal Property (§1245): $500KOriginal Basis Allocation:Building: $1.6M (depreciated over 27.5 years)Personal Property: $400K (fully depreciated)Building Depreciation:Annual: $1.6M ÷ 27.5 = $58,182Total over 10 years: $581,820Gain Breakdown:Building (§1250):Sale Price: $2,300,000Original Basis: $1,600,000Less Depreciation: ($581,820)Adjusted Basis: $1,018,180Total Gain: $1,281,820Unrecaptured §1250: $581,820 (25% max rate)Capital Gain: $700,000 (20% max rate)Personal Property (§1245):Sale Price: $500,000Adjusted Basis: $0Ordinary Income: $500,000The Strategic Play:Remember those suspended passive losses you couldn't use?

Charge the tenants a higher rent rate and include utilities with their rent.