Unfortunately, since you want to keep the existing house, the banks would normally look at the 2nd as an investment mortgage. There might be cases where they would treat differently, but I understand they will say you can only have 1 PPR at a time.
Re status of property, you should get planning permission for change of use to residential, if that's what you need, before you buy. Banks might ask for qoutes for any immediate work needed to convert to residential.
You haven't mentioned cost of new property, or value of existing house, so can't comment on financials.