If you are financing a home you already own, most banks will want at least 20% 'down' (i.e. 80% loan to value) but this may vary considerably between lenders. If you are purchasing a home with 2 loans the requirements will vary even more, though nowadays it is usually better to go with a single loan. You might also consider refinancing your first mortgage for a higher amount to get the cash you seek. At today's low rates you may save considerably on your monthly payment. At any rate, you need to talk to some lenders to get the answers for your particular case.
If the Buyer has no Buyer Representation Agreement with the agent and the agent appears to have dropped him/her, another agent can certainly be hired to take over. Theoretically, you could continue with the same offer but due to the rather inflexible REO policies it would be much cleaner to cancel the offer and submit a new one. Be aware that, If you have a verbal acceptance of the offer, cancelling it may open it up to other offers. The new agent should discuss this with the listing agent.
Veronica2,The Zestimate function is a very imprecise tool. It is a good place to start but you need to obtain a more accurate value. Most Realtors offer a free Current Market Analysis if you are thinking of listing your home for sale. If the value is more than your mortgage payout plus closing costs your loss may be more than offset by finding a good deal in your new purchase, provided you shop carefully and negotiate well. If the value is less than your mortgage it's more difficult. Most banks will not allow you to do a short sale and get a new mortgage right away. For Fannie Mae loans you will need to wait 2 years. One option would be to by a home and rent your current home until it can be sold, but some lenders will resist the new loan under the suspicion that you will walk away from the old one. If your own lender is not helpful your Realtor should be able to refer you to lenders that may have a more flexible policy. Often the small local lenders are able to take a more personalized approach.