If you come in after that, we cannot guarantee a timely filing (top secret, and only on BiggerPockets, we bake in some flex in our system and can likely file on time).All our extensions are client requested.

If you decide to use leverage you have to use non-recourse loan since you are considered to be a 'disqualified person' and therefore providing personal guarantee with conventional loan would not be allowed.The $50K or 50% of the balance limit you are referring to is applicable for a Participant Loan - that is if you personally need cash you are allowed to borrow from your 401k using those guidelines.

I would vet them intensely and make sure the lease is personally guaranteed. 

Is the builder guaranteeing these prices/budget (a fixed price contract) or is the builder cost plus a percentage? 

Don't throw all your money into something that's not guaranteed to work out

(Since they will actually be in a worse position then than not selling.) they should ask for 30% or more down, which will give them 20%. 10% of that might be required to sell again. 2) if you can earn 5% or more guaranteed in the bank, what would you require from a stranger where it will cost you 10% to take back the loan and resell to get your money back?

They guarantee that the buyer of their program will buy a specific volume of property at specific minimum terms during the next 12 months3.

In the worst case, I've seen contracts where LPs are made to personally guarantee the loan.

Of course you could put it a guaranteed bank cd and make over $300k so make sure the juice is worth the squeeze.

This might mean asking for personal guarantees or using assets like personal belongings, investments, or other valuable items as collateral, aside from the property being bought.