I bought a property in DEC 2018 and have since proceeded to fix it up with intent to rent in 2019. How do I document purchase and capture 2018 rehab expenses? Or do I?

The expenses become part of the basis of your rental property. In fact, all expenses in 2018 and 2019 before the property is "placed in service" are capitalized (see next paragraph). Placed in service means that the property is available to rent, not that there is an actual tenant.

If you are in the business of renting property (i.e. generally a full-time job), then you can deduct up to $5,000 in start-up expenses in the first year).

The basis (less depreciation) is subtracted from the sales price (less sales expenses) when you sell the property, thus reducing your gain.