@Sanjoy V.
A few things I want to comment on...other points have been well covered. First, you have to be careful who you listen to. The tax guy is giving you a lot of fear. If you've done your homework and know your numbers, his fear and criticisms will be of no use to you. When I first started investing, I was told by people I would lose my shirt. A lot of people have fear about taking chances. I applaud you for taking chances.
Also, $50.00 to $100.00 per door under market is like not being under market at all. I look for value add, especially in a C property. If you can only get another $50.00 bucks, how is that a value add?
The assumption that the property management is good is a risky proposition. I've not yet seen a good PM, even when other property owners say the PM is good.
Last, dig much deeper. You haven't mentioned what the building was bought for last. Nor, have you mentioned that you have seen the lease roll. How do you know the seller didn't stick bodies in the units with awful credit, with great deals, to make the building look more occupied than it really is. And finally, throw out the pro forma. Pro Forma is science fiction in my book. Lastly, this is a $6 million deal only if you pay $6 million. The price is not $6 million. The price is what you will pay for it.
Also, ask someone else you trust, or someone like @Jay Hinrichs to examine the deal, after you have all the information. I have a good friend who asked me to review a 16 unit building he wanted to buy. I waded into a ton of data, and told him to run, and run he did.