(1) is your most important question. The receiver battery needs to be checked after about three flights. You buy an inexpensive "loaded voltmeter" also called "Expanded Scale Voltmeter" (ESV) - about $20. It is a voltmeter that puts about a 300 mA load on the pack. When this shows 4.4 volts you are done. You hook up your switch / charging harness such that the charging jack is external to the fuselage. This way you can check the voltage without removing the wing. Most receiver packs will give you at least 6 10 minute flights. But this is dependent upon the number of servos and how hard they are working. The servos use much more current if the surfaces are binding (e.g. hinges are stiff).

(2) NiCd's ares still used but they are not as common as they were 20 years ago. The whole issue of partially discharging causing a battery "memory" is largely unfounded. And there is no such problem with NiMh batteries or Lith-Polymer or Li-Ion or LI-Fe. I would not worry about it. Just charge your packs up with the provided charger for 10 hours the night before you fly. There is a value in owning a "cycler". You want to test your battery capacity about every 6 months. So you charge it up, and then discharge it using your "cycler". The cycler measures how much current flows out of the pack until it falls below some low-voltage cut-off (e.g. 4.0 volts). If the batery has a capacity of 600 mA when new, you will find that after several years, it will show less capacity. If it gets down to 80% of the original, then it may be tinme to buy a new pack.

(3) Field chargers are generally not needed and are hard on batteries. They much prefer to be slow-charged with the wall-wart charger. However, if you fly more than 6 flights per day, you may need a field charger. Buy one that has a cycler in it and you will have two devices in one unit.

(4) About $100 will get you a nice charger / discharger. Then you will need a 12 Volt battery to drive the field charger. You can use your car battery if you park close to the flight line.