Some of them I have had awhile and now am able to command higher rents than when they were purchased. 

I have a lot of market knowledge here because of my sales history with other clients so we didn't have to use real estate agents in a traditional way and I handled all of the paperwork.

Adding/removing users is tricky.

@William GravesBefore walking a house, I estimate using rough $/sqft from previous experience.If that gets me near a purchase price that I think can be negotiated, then I'll go visit the property.Generally when I first walk through the property I estimate by taking notes on this spreadsheet:https://www.biggerpockets.com/files/user/kubaf/fil...While I'm there I am also using my smartphone to record the entire walkthrough, because I always refer to that video later during itemization.Then I go back and rerun my numbers.If the numbers are still in the ballpark I'll make the offer.If the offer is accepted, at that point as part of the property due diligence I will then spend the time to itemize the rehab costs and solicit bids based on the itemized scope of work.

I use Propertyware and find it much better than Appfolio but every user is different. 

Typically, a nicer house in a nice neighborhood will both command a higher rent and a higher purchase price.

Too long for here so go to my Profile https://www.biggerpockets.com/users/MikeM500 and follow the link.

I do not think however that the economics work well, you are unlikely to cover your expenses as what you can command in rent may not cover a mortgage due to high real estate costs.

How are other BP users finding the number of units on a given apartment building when doing due diligence?

If there are no rents, then the appraiser will use rents from comparable properties, which may not work to your favor (never does for me).