Supposing I get the average rent per room this way, I could either just multiply by 4 to estimate rent for the 4/2 condo, or I could subtract some on the assumption that the average student bedroom rents for more than each of mine will.

To calculate cash flow, you need to do two things I don’t see here:Make an assumption about the percent vacancy, and adjust the annual rent accordingly.

I've found that these are decent placeholder values as starting assumptions, but you can refine them down the more you know about the property.

I do feel your return assumptions for buy and hold are too aggressive.

I know what it looks like on paper (no pun intended) but always enjoyed testing my assumptions.

You're making all of these assumptions about loan amounts and a loan for this and a loan for that and you haven't even spoken to a lender that knows the area, the Fannie and Freddie loan maximums, what the down payment requirements are or why or why you wouldn't take a 15 year over a 30 year.I don't know what you do for a living, but you're making a ton of assumptions about an industry that is really pretty intricate and in the pool you're thinking of swimming, has a massive amount of overlays meaning one lender will have a product that another doesn't have and another lender has a twist to the same product because their secondary market guy is willing to assume a little more risk than the other guy.Call around to local lenders for what you're trying to do. 

@Rene Bee, I defer Matthew regarding 203k loans, but I was under the assumption these are for owner occupants (not the case here) and with minor rehab needs ($30k max).  

and thats just the stuff i know about. inspection is next week.their rent is 650 now and thats under market. after repairs i should be able to get 750 (maybe upwards of 800). it blows my mind that anyone would rent under those circumstances. that could be good and/or bad as far as having them as a tenant. their current lease is month to month and is a halfway-filled-out basic (garbage) lease.under the assumption they can clear screening and put up money for deposit/last month and everything else looks good on paper, would YOU let them stay?

@Alan Brown, I was making an assumption that the 1031 had provided extra funds. 

So it takes about 45 months to recapture your costs to update and retenant...that is if the new one stays for the 45 months and your lease assumptions are correct.Depending on where you are in DFW, you also might consider trying to get the lease on school year.