Hi Andy - one thing you won't be short of here is opinions - mine follow
Image Stabilisation - I've never had image stabilisation in any camera body I've had, only in a 400mm telephoto lens I use for long-range shots, where it's really useful. In order to get get the stable image I need when photographing butterflies, I think a fast shutter speed, flash (if necessary) and a tripod are as (if not more) effective (since you'll still need these even if you have image stablisation!). In summary - this is a "nice to have" and not a "necessity" for butterfly photography.
A Live View screen - A complete and utter waste of time in my opinion. All it does is drain the battery more quickly.
Dust Reduction - Another "nice to have" since it's not "dust elimination"
"Aspect ratio of the sensor 3:2 or 4:3" - I'm not sure about this - I always thought that the aspect ratio of most "enthusiatic amateur" DSLRs was the same - it was the magnfication that changed; I'm only aware of a full frame sensor (1:1) or one that gives you a 1.6 magnification. In which case ... the answer really depends on the lens you intend to use in combination with the camera body. I think a full-frame sensor with (say) a 150mm or 180mm macro lens will give you more flexibility when you want to shoot landscapes (where the 1.6 magnification is a negative feature!). A sensor that gives you a 1.6 magnification with (say) a 100mm or 105mm will result in a dedicated setup that is less-flexible (which, by the way, is what I have!).
"It might be good to do the occasional large print of 10 x 8 and not lose too much in cropping, is it correct to say that you don’t lose too much from a 6 x 4 when using the four-thirds system?"
No idea. See above
"Can one get 9 x 6” prints from companies such as Snapfish, and if not why not? !!"
I've never heard of Snapfish.
"Models I am considering are Pentax Km, Nikon D40/D60, and Olympus E420; plus perhaps the Canon 1000D. I haven’t yet handled any of these cameras. It’s very important to me that camera and lens are small and light, as I’m a fairly small chap with small hands!"
One thing to bear in mind is the range of lenses you'll have access to. The camera body is only a small part of getting a good image and some of the lenses are really heavy Anyway - I'm sure you'll be able to handle any model you buy - so long as you use a tripod when necessary (which is what I do when I have my 400mm lens attached!)
"As an example of a situation, if I am 5 or 6 feet away from an insect on a plant stem, and I know I can’t get any nearer because the creature will be disturbed and fly off if I do, what sort of lens in mm, would I need to get a detailed frame filling shot?"
At 5 or 6 feet away, probably a 400mm lens!!! That's a very long way away!
"The past two years I’ve been using a Canon Powershot compact (with a 6 x zoom) and it’s exactly this type of scenario where I couldn’t hope to get a shot; although if you can get within a foot it will do quite good shots in macro mode."
I use a Canon 30D with a 105mm macro lens, and get decent shots around 3 feet away.
"I’ve heard quite a bit of praise for the Sigma 105mm macro (F2.8?) lens, but, bearing in mind the above example, from how far out from the subject can it be used?"
See above. Some on this forum would insist that a 150mm macro is the best option.
"Would a kit lens 18-55mm be any good in this example, or would I need to buy something else?"
You need a dedicated macro lens, in my humble opinion.
"How about purchasing an 18-105mm lens as a general purpose, including landscapes, plus a dedicated macro, eg the Sigma 105mm macro F2.8? Maybe I could get away with just two lenses then!"
My 2 main lenses are a Sigma 105mm macro, and a Canon 100-400mm telephoto (mainly for birds and long-range shots). For general-purpose portraits, I use a Canon Powershot G9 since my camera body doesn't have a full-frame sensor.
Cheers,
- Pete