The balance on the mortgage is a little less than $40K and the value is around $175K.

I then look at the balance being foreclosed on and how old the note is to get an idea of whether the house is financially underwater.

So when sellers state that they don't want to tell you details about their property they want to sell (ie, the mortgage balance), and they just want an offer.....what do you tell them?

This will invaribly be different for each rate and scenario but just a general gauge.Even though rates are in the 3's and 4's the monthly payment or what we call the "mortgage constant," on our end is higher because your payment does not consist of just interest, but interest and and 1/360th of principal on a 30 year fixed loan which is designed to amortize/payoff to $0.00 balance by year 30 or 360 months.Typically a rate around 4's will have a monthly mortgage constant of around .50% monthly (6.00% annual) or about $500 per month for every 100,000.Sometimes when the rate gets into the high 3.50% range the MC is around .45% per month ($45 per month for every 10,000 borrowed) so it doesnt vary much while at 4.50% 30 year fixed the MC monthly is around .507% or $50.70 per month for every 10,000 borrowed.I suppose all this technical speak means is that if you borrow money where the outflow out of your pocket is around 6% with interest and principal considered then you'll probably want to reinvest it somewhere else where you can get considerably higher rate of return on this cash than 6.00% atleast this is how I look at it.If you want to make 6% + 6% for cost of funds then you'll want an investment with a hurdle rate of 12.00% or higher cash on cash as an example.

A couple of questions:1) Are you borrowing $100,000 for the down payment on a property and securing conventional funds for the balance of the purchase price?

So wondering if I can withhold the balance of the SD until a new tenant is placed?

It's a balance.

Ocupant owners of mobile homes have way too much debt to buy even at just their un paid balance.  

Option to payoff principle balance before balloon period with a fee of $2,000. 

If the seller or tenants refuse to do this, then you can ask the seller to put 45 days of PITI + expenses in escrow with the remaining balance refundable back to the seller if the tenants move out earlier than 45 days (this however, does not guarantee that the tenants will move out within 45 days but it does put pressure on the now seller/landlord to push for option 1 - having the tenants sign something).