Well, look at the numbers:
$639K Purchase Price
Assume 0% Down
30-year loan @ 8% = $4688.76 loan service AT your listed NOI that is a positive cash flow of $1245.49.
Assume 10% down = finance $575.1k
30-year loan @ 8% = $4219.88 loan service AT your listed NOI that is a positive cash flow of $1714.37/mo
Assume 20% down = finance $511.2k
30-year loan@ 8% - $3751.00 loan service AT your listed NOI that is positive cash flow of $1283.27/mo.
The thing I would be very careful of is your expense ratios. You are assuming about a 35% expense ratio - which will include all taxes, owner paid utilities and other expenses, management, and most importantly, vacancies and short and long term maintenance programs. Many people end up getting lower expense ratios like this because they don’t fail to take into account long term maintenance requirements - you have to replace the roof every 7-15 years - replace the carpet every 3-7 years, repaint every 2-4 years, replace the furnace/AC every 7-15 years, restripe the parking areas every 1-2 years and resurface the parking areas every 3-5 years, etc, etc,etc,etc. So what happens is your budget is enough to handle the odd clogged sink or broken window, but then you have to replace all of the rotting stairs and it busts your budget.
Generally, the newer a property is (or it has had a recent, 100% rehab) then it is possible to shrink the preferred $50% assumed budget - but again you still must budget monies for those high ticket maintenance items some time in the future. It is better to start budgeting them now so when they start coming up in a couple of years you have a cash balance to help pay for them.
If it is an older property and several years past new, realize that many high ticket long term maintenance requirements are likely to be right over the horizon. Where will the money come from - a new loan that kills your positive cash flow anyway?
Make sure your vacancy, management and maintenance budgets are using realistic numbers.