The more you leverage the property (aka with seller financing), the higher the risk that you take during a downturn in the market because your cash flow is less than when not leveraged.However, if you buy at a deep discount, leveraging is your friend because your risk is already diminished due to the low purchase price.

Would a bank require a security interest in the marketable timber, requiring me to give them the $30,000, due to the harvest of the timber and possible diminish in value of the land? 

I have noticed around here in CT that the amount of my discounts have diminished. 

They diminish yearly so you can check the loss every year on line by going to NADA Used Trailer guide and see what they are worth.

Additionally, the quality of renters diminishes as the quality of the MH's decreases.Most importantly, however, old MHC's have old infrastructures and that's where a lot of newbie MHPO's can really take it in the shorts.

You just buy the rate down, even into the absurd diminishing returns range, because it isn't my buyer-clients money, it's the seller's money, and I'm perfectly happy to waste the seller's money if it gets my buyer client even a slightly better deal.

there seems to be a great many homeowners where values were way up a few years ago that just haven't accepted that the value is diminished quite a bit.

:)So I can come up with the whole 348,050, but it will diminish my cash savings to the point that I may not have a contingency fund in case of emergency for many months

Also in regards to continuously putting down 15% - 20%, at the time I was purchasing the property was before I had discovered the BP podcast and was not aware of all of the more creative ways to purchase properties and my only other known alternative at the time was to use an FHA loan on which the mortgage insurance would have diminished the return so much that it wasn't feasible. 

A seller will sell best case scenario and diminish greatly the chances of anything bad happening.