If a seller refinanced a house at around 220k and has about 120k left in mortgage, but the house also appreciated to 450k, how do I structure a subto/seller finance deal?

It's highly doubtful, you prob won't find a fully refinanced deal on the MLS or through realtors. 

Cash out refi only if the rental income significantly exceeds the monthly payment and you want liquidity to invest elsewhere.2.You can maybe do a HELOC if you want a smaller loan that doesn’t involve refinancing the entire house.3.

The property jumped another 100k less than a year after the sell as well as rents went up to support what would have been the new monthly debt if I refinanced and held. 

I am in the process of refinancing.

Consider increasing rent, refinancing, or switching to a mid-term rental to improve cash flow.

With rapid appreciation and cash-out refinancing, you will only need a small portion of that amount.Low risk of natural disasters.

There is no point refinancing the almost paid off property to buy another-just get a mortgage on the new property.hard money lenders are going to charge you a higher interest rate.

You may be better off refinancing the properties you do own to accomplish things.

You should be able to go as low as 10-15% down payment with your experience.Ground up Financing: Most "ground up" financing options require a bit of experience - 3-5 exists (rehabbed & sold or rehabbed & refinanced in the last 3 years).