The Sennheisers are very good value, and perform well. Anyrthing cheaper isn't going to sound right - bearing in mind the SM58 has a fairly large diaphragm and can handle some serious sound pressure levels. A small electret capsule will tend to make some very odd noises if you speak directly into it, which is why the cheaper ones have a big foam windshield.

You'll need to experiment a little with the position of the microphone until you find what works best for you.

Don't be fobbed off with any VHF stuff - hardly anyone uses them now, because UHF is less troublesome.

However, a lot of people are surprised when they get a loud burst of interference when they switch off the transmitter, leaving the receiver open... it isn't a fault, that's why transmitters have a mic-mute switch!

As long as you are sensible about 'rotation' of re-chargeable batteries, you'll find that Nickel Metal Hydride (NiMH, at least 2000MAh for AA type) will give you a good compromise between cost and battery life. NiCADs are not intended for such use, and will not perform well.