Hi Jonathan -- just a few things that have sunk in for me in my long illustrious (6 mths and counting!) investing career:
You should multiply the Gross Potential Rent by 5% for "Replacement Reserves" and include it when computing your Net Operating Income. You will need to fine tune the 5% by looking at the building mechanicals and roof and estimating remaining life of these items and replacement costs. If replacement is needed in the next couple of years on the roof, for example, and you elect to not do it immediately, I normally include that item in the up-front purchase price (for analysis purposes) and recompute the ROI in that fashion. In this case, you will need to reserve funds (or build funds) to prepare for that replacement.
An advantage of knocking out these "near-obsolete" items up front is that if you're financing with a community bank, the bank will often loan you funds for "purchase+rehab" for all the work you do right around the time of purchase. Then you have a functionally new property that will require less maintenance and be much easier to sell if need be. And.. if you do find yourself needing to sell for some reason at a later point, you may or may not have funds at that time to replace a roof and other items, forcing you to dump an "as-is" property at a loss.
I'd recommend using at least a one-month vacancy factor (8.3%), and be sure to add in the prop mgr's cost of leasing the unit (commonly half month rent). So if the unit turns over every year, prop mgmt could be 10% (normal fee ) for 11 rented months, but 50% in the vacant month. This equates to a PM fee of 13.3% for the year. You'll also have to clean, paint and possibly replace flooring at these turnover points, so make sure you include this in your PM fee or add to maintenance. If it's a multi, don't forget yard maintenance, snow removal, etc., even if you're doing it yourself. (time is money).
If you're buying well under tax-assessed value (usually the case), make sure you research and understand the process for appealing the assessment, and how long this will take. You can save a ton on this. There are services that will handle this for you for a % of the first year savings, or a flat fee.
You'll be buying way under "replacement cost", so determine how much you want to insure (perhaps 50% above your all-in cost), and base your insurance calc at that level (perhaps .40% multiplied by 150% of all-in). Don't let the agent convince you to insure at "replacement costs".
Be sure to add in loan closing costs to your up-front. Some is fixed -- lender fees, appraisals and inspections (these are higher for multi-unit), title search, attorney fee -- and some is variable with your loan amount -- points, title insurance, county fees sometimes), so I like to break it out in that fashion.
On a side note, purchase an extra umbrella liability policy to protect your property equity and personal assets from lawsuits. 1mm should cost $200-250/year.