A hydrometer isn’t needed for OG when you brew an extract kit. The OG will be as specified in the recipe as long as you use all the fermentables and the volume in the fermentor is the 5 gallon recipe volume. You will need a hydrometer to determine when the SG has stabilized and the fermentation is complete. With some yeasts this could be a short as 5 days. The beer is still not ready to bottle when active fermentation is complete. You need to give the yeast time to clean off natural off flavors of the fermentation and then time for the excess yeast and suspended particles to drop out. This will reduce the amount of sediment in the bottle. Three weeks is a good time frame to shoot for. At three weeks the SG sample should be almost totally free of CO2 and the sample should be clear. CO2 in the beer will keep particles like hop debris suspended which if bottled to soon will add to the sediment in the bottle.
Add a stick on thermometer strip to your fermentor for tracking the actual fermentation temperature. Yeast produces heat as it works. During active fermentation the temperature of your beer will always be higher than the ambient temperature. High temperatures for most ale yeasts will produce extra esters. Very high temperatures will produce fusel alcohols.
Effective temperature control can be a simple swamp cooler or temp controlled frig or freezer. This is a picture of my very simple swamp cooler setup.
