On the eventual disposal of the properties you will have to consider the capital gain tax (CGT) consequences. This tax will be levied on the difference between the proceeds/market value and your capital costs. It is therefore prudent to keep a contemporaneous record of these capital transactions. For each property you should record.
1. The full address.
2. The dates the property was purchased and cost or the date the property was inherited and the probate value.
3. The expenses of acquisition.
4. How the purchase was financed, lender, amount borrowed, type of mortgage.
5. Particulars of any capital costs (new kitchen. Bathroom, roof etc).
You can also claim a (generous) reduction if any of the properties were your principle private residence at any point. If this applies keep a note of the dates.
On our rental accounts we include a balance sheet which shows the capital history.