I think you shouldn't have a problem with regard to equity as you have 50% available on your PPR.
You should get a local estate agent to determine the likely rent you could expect. They'ill put it in writing for you. This can be used when negotiating your mortgage with your mortgage provider.
One thing which you have to be aware of is that if you rent your original house and move into the newly purchased property, you will not be entitled to offset mortgage interest against your rental income when calculating your tax liability.
If you do it the other way round and simply buy an investment property and rent it out, then you will be able to offset the investment mortgage interest against your rental income.
It's something you will have to factor into your deliberations