Yes if you have come to some sort of agreement about what should happen if you split, its important to get it in writing, legally. By having a prenup, you can make sure that your assets and liabilities will be apportioned according to your wishes as prescribed in the prenup and prevents the court from being involved. Make sure you see a solicitor to draft it for you as a prenup must comply with strict legal requirements and you will need to have a certificate from a lawyer that says you have received proper advice. If the agreement is not done properly it may not be legal and may not hold up in court.
A prenup suitable for a situation like this where you are bringing property into a relationship that you want to be able to take out with you as well. It can also provide for a number of other situations which you should speak to your lawyer about. If you are getting one drawn up you might as well make the most of it.