The conversion is simply a couple adapter pieces that go on the high and low fittings. You would need those, the proper compressor oil, and freon. Then hopefully the system doesn't leak and functions properly.
The gauges are the same, it's all about the fittings. I have gauges, but would need the fittings. Orifice tubes are inline filters, and cause most ac issues to begin with so yes I'd change it (if there is one) and the dryers are low dollar and can clog up as well. Most parts stores won't warranty compressors unless you buy those at the same time.