What numbers or factors do I look for to say, "this property is gonna be really good, go up in value, cashflow nicely when my roommates move out in 2 years."

There is a price premium for having a backyard, not sharing walls or even allowing pets that just can’t be factored in with that site. 

If it takes much more than 10 years to pay it back (after factoring in my normal cash flow) I don't consider it.

Of course there are other factors to consider as well when deciding on a property such as location, stability of the area, crime, etc.

Personal credit will be a factor when applying for sure, however you should be able to find a loan product that is non reporting and will not affect your personal credit going forward

But again, it's going to depend on factors specific to your situation.

You would also need to weigh the risk factor for each investment.

If the market tanks (rents are currently in depression while prices remain at all time highs in my areas) it would be for different factors, at least in my opinion.

Not sure the type of return you are expecting but with the purchase price and units provided definitely make sure you account for all factors prior to moving forward if you choose to.

This is not your best option as then each unit is tied together and an exit strat becomes more time consuming and difficult to do because your lender is now the deciding factor in splitting the loan apart if you choose to sell.