Most lenders look favorably on this situation. As a rule, fixed rate mortgages are not assumable. Banks don't want to continue to be locked into a low rate. This may make assuming the seller's mortgage less attractive than getting a new loan. In the case of a new home, the seller is the developer.
Developers frequently offer attractive financing to sell homes quickly.
Perhaps someone you know could be persuaded to make you an outright gift or loan to help you pay for your home. A loan offers advantages to both parties. The friend or relative can usually earn higher interest than on a savings or money market account. You would pay less interest than to a commercial lender. Make sure the terms and conditions of the loan are clear to both sides.