You'll need 20% of the purchase price plus your closing costs and prepaids.  

Your lender may require more. 5% above your down payment and pre-paids/ closing costs.   

@Christopher Dru- thanks for the  post ...1)  30 yr fixed rates for PRIMARY  SFR are around 7% right now -  rates for a 2 unit duplex thats a rental are  signficantly higher  right now ...2) also  worth noting - the points  ( loan fee ) for many rental loans are  very high  right now   so  make sure to get an  accurate picture of these  plus all other costs and  prepaids  to  plug into your  calculations    3) rates  are  likely going to  eventually  drop  but  dont make any decsions  based on hoping rates  drop    4)  private  financing  terms will likely be  higher than traditional loans 

If someone has prepaid the mortgage, the most likely want to pay off the mortgage early, not move money from a liquid state to a less liquid one.

Unless you can find a prepaid service where you don't have to attach your name to it then I wouldn't mess with it.

As for your options now while your credit is suffering…private money, hard money, seller finance, prepaid PMI, assumable mortgages and sub to all come to mind.  

i think a decent SF will cost 150k.and with 5% down n closing cost and pre-paid amount it will be way nore than that.

If you use it to pay future mortgage payments, which it sounds like you are proposing (5 months), then you did not avoid those interest payments, you simply prepaid them.