Question on obtaining financing-preapproval first or run numbers?

[Eric Grant]

My fiancee and I are looking to purchase our first investment property. I have already used my VA loan for our primary residence and she was recently turned down for an FHA loan due to her debt to income ratio being to high (student loans). We have capital to invest so now we need a lender. Because of her being turned down we are considering going the conventional route or possibly commercial for a 5+ unit. My question is this; should we go for a conventional loan for a 2-4 unit and try for a regular pre-approval or go to the commercial side for a 5+ unit and if we choose to do the latter should we run numbers on a few properties before looking for a loan? Clearly a conventional on a 2-4 would give us parameters to work within but I'm not sure how to go about looking into the larger buildings in regard to financing, knowing that they take income generated from rents into account.

I hope that my question has been asked clearly. Thank you in advance to all who respond.