@Mar Now
If you apply the 70% rule to the multifamily, the max you would offer would be $377k. So buying it for $450 is pretty far above that.
The other problem I see is neither of these properties reach the 1% rule of monthly rent to purchase price: 5,200/710,000 = 0.73%, 1,700/350,000 = 0.49%. Refinancing either of these will kill the cash flow. You can only do about 65% LTV on the house before you hit negative cash flow, and about 60% LTV on the multifamily.
Pulling 75% LTV out of the house with the HELOC will lead to negative cash flow, which is why @John Leavelle suggested this as only a temporary situation that you pay off as quick as possible with the refi of the multifamily.
Maybe a better way to look at this decision is this: You have 1,250 monthly cash flow coming in right now. If you sell the house and walk away with $325k after costs of sale, can you then invest that money in other deals to generate 1,250 OR MORE per month cash flow?