Hi Judy,
I'm certainly no expert so please don't take this as any more than what I've discovered works for me (and sometimes doesn't work).
You probably don't need me to tell you that the most important bit isn't the kit at all, but the proximity of the mic to the sound source. A cheap mic inches away from the subjects mouth will sound 1000 times better than an expensive mic several feet away.
Assuming you don't have the luxury of a sound recordist and boom mics this means either lapel mics or hand held mics. The latter is only relevant if you don't mind the mic being obviously on show and if your interviewer/interviewee can be relied upon to use it correctly (ie only speak when the mic is in position). Most shotgun mics aren't really suitable for this unless you have decent pistol grip with shock mounting as they tend to be rather microphonic. Hand held mics are constructed to reduce handling noise, but unless you are planning to do lots of this sort of work, probably not worth your while buying.
However, if you know people in bands, they may have suitable handheld mics you could borrow. The "singers staple" the Shure SM58 can give good results but really needs to be held close to the mouth - possibly too close for a pleasing looking video.
For the pulpit, a shotgun mic on a stand may work well so long as the speaker doesn't move around too much (like I tend to whenever I speak).
This leaves us with tie-clip/lapel/lavalier mics.
Most are designed for speech, as you'd expect, and respond well enough at those frequencies. The difference in price is really down to sensitivity and the amount of background hiss. Like most things there's a law of diminishing returns. I have a little ATR350 now superseded by ATR 3550. It cost me about £17 and is fine. It produces hiss, but this is easily removed with Izotope RX (expensive, but the piece of software I use most in video making aside from my NLE) and I find no trouble getting acceptable results.
I normally pair this with my Zoom H2 digital recorder. Again with digital recorders, one of the key factors in the price is the quality of the pre-amp and a cheap digital recorder will most likely introduce more hiss than an expensive one. But the hiss isn't too bad. It introduces far less hiss than the amp in my arguably much more professional Canon XF100. Worth checking this our if you plan to run a lav directly into your camera - you may find a separate recorder gives better results.
If I was buying a digital recorder specifically for a lav and I wanted it cheap, I'd probably get A Zoom H1 (wasn't available when I bought my H2). Many people will argue that the Tascam recorders have better pre-amp quality at this price range. As you go upmarket the Marantz PMDM 660 (I think?) is highly rated.
For your two-person scenario, if your digital recorder has a stereo 3.5mm mic input jack, you could buy an adaptor to allow two mics to plug in - one on each channel. This has the advantage of only requiring one recorder and also means you will have only one track to sync with the camera's audio.
The biggest problem with a lapel mic attached to a digital recorder, at least if it's being "worn" by a distant subject is you have no means of monitoring it. Imagine you've wired up the Mayor for his outgoing speech to the Council, tested it and checked for levels, then whilst he makes his way to the Council Chamber the jack plug falls out of the unit, tucked away in his pocket. You have no idea this has happened until after the event. This happened to me two weeks ago. Yes, in retrospect I should have used rubber bands/tape whatever to secure the jack, but then again I needed to do a quick changeover from him to the incoming Mayor. Fortunately I had another recorder with inbuilt mics immediately in front of where he was speaking (and - for worst case scenario, another digital recorder in the middle of the chamber). It's all about redundancy!
Wireless means you can receive the signal where you are which means you can monitor it - either through your camera or your digital recorder (remember - this may have better pre-amps than your camera). However it's expensive and most serious amateurs wouldn't go for anything less than the Senheisser G3 system (>£400) and that's only giving you one mic.
UPDATE: I forgot the recently release Rode system which has had good reviewes, and now this:
https://www.youtube.com/watch?v=z0SOhCrnB8U. I have no personal knowledge of either though ... but must be worth investigating.
However you go about it, there are many potential points of failure that you cannot see/monitor:
battery in mic (if powered),
connection from mic to recorder
no monitoring (actual sound much higher or lower than when setting levels)
mic becoming unclipped/falling off
battery in digital recorder
battery in transmitter
battery in receiver
For your "round table" discussion, the problem is the proximity, but I'd attempt it with a digital recorder with built in Mics. Try to isolate the recorder from the table by putting it on a suspended stand or bean bag, as it may well pick up the slightest touch.
The choir may well also be adequately served by a digital recorder and inbuilt mics. Incidentally, here you are NOT trying to get the mic extremely close to the source otherwise you will "feature" some singers over others! I'm looking for an opportunity to record such an event with my recently acquired Zoom H5 using internal mics in conjunction with a pair of shotguns. this would give me four tracks and either a choice of mics or the opportunity to mix all four. I bought this to record a school production where all the sound was through a PA. I used the XLR connectors for the audio direct from the sound desk, but this was too clean on its own and I'd take the precaution of recording auditorium sound through the built in mics as well so I could add some ambience back in.
Hope this helps - if only for others to point out where I'm wrong