I got great advice from a very experienced rescue diver when I had the same questions. If take diving seriously you will progress rather fast and outgrow your equiptment, try to buy advanced recreational stuff so you don’t have buy again quickly.
You should invest on an air integrated computer, it does measurements based on the actual air inside the tank and your breathing rates. Two types: 1- Wireless: wrist style and transmitter connected to the High Pressure port on the first stage they usually run between $600 to $2000. 2- Connected: Gage style connected to the high pressure port of the first stage through a hose they ussually go for $500 to $1000. I have the Suunto Cobra I payed $650 with the computer module and quick disconnect. I couldn’t be happier. It is very accurate, easy to read and operate even at depth. It’s nice to have all the info together.