Mortgage Advice for a Short Term

glp

I have a property just going on the market which has a small mortgage.
I own a tiny cottage which is also on the market.
I have made an offer on another house and has been accepted. If both my houses sell, i will have an amount over the purchase price of the new found property.
I have been advised to change my existing mortgage to a buy to let until the house we are living in at present is sold, which will pay off the existing mortgage and give me my deposit for the new property, and rent out in the meantime if necessary, then at the same time, get a new mortgage for the new property.
I will only be able to afford interest only mortgages as they will be running together.
Do you think this is the best way of raising the deposit and being able to buy the new property, knowing that i would pay off virtually all the mortgage once the money is available from the sales.
I am bogged down with all the different mortgages available to a point i really dont know enough for the best.
Thank you.
Gillian