2. There's no simple method to add a readout to float-type flowmeter (rotameter). Most economic flow meters (I doubt you find cheap sensors with suitable precision) have thermal or pressure drop operation principle, also some simple turbine flow meters are on the market. Depends much on the intended flow range, precision, gas type.
For some applications, a rotameter with light barriers can be used as limit switch, e.g. for min/max flow.
3. Consider if you want to measure pressure below actual atmospheric pressure (differential measurement) or absolute pressure (uses vacuum reference). Depends on your application, both sensor types are available.