Several things...
1) Radio slaves for Canon flashes...
Pocketwizards - Most reliable, great range, most expensive, MANUAL ONLY - you need to preset flash levels or play with many flashes to remotely set them from a handheld flash.
Radiopoppers - New, seems excellent, enables you to shoot ETTL still... Cons: requires a flash on the camera (or wireless flash transmitter) in ADDITION to the radiopopper sender unit. Not strictly available in NZ but that shouldn't be hard to work around.
Generic manual radio triggers... These vary in quality and price. I had a set of Gadget Infinity triggers (Often called Ebay triggers) and they worked ok but had bad range and awful build quality. I upgraded to Flashwaves. EXCELLENT build quality and about half the price of pocket wizards for similar functionality (but range not as good).
You can also use Canon's own multiple flash system but it requires a 550EX or 580EX on camera, and a similar (or 420/430ex) remote flash. Since it uses IR and visible light to work, you need line of sight between flashes and camera, and it can fail to work in bright sunlight.... BIG upside is that you can control everything from the camera and use ETTL.
2) 1.4x and 2.0x extenders
I bought 3 off brand 1.4x extenders for Canon (a sigma EX, then a Tamron and finally a Kenko Pro)... I used them all exactly ONCE before onselling them. They affected sharpness and focus accuracy noticably. However the chance of a 1.4x telephoto adaptor is too hard to resist so I finally gave in and bought the Canon 1.4x extender NEW (The ONLY bit of equipment I have bought new except a 50mm1.8 and the flash triggers!) with the intention of using it for one specific event and reselling it... However when I looked at the images I cannot tell which are taken with and without the extender so I had to keep it.
Sigma / Tamron etc make some very fine lenses (I have owned / own both and there are some gems) but for me, there is no good alternative to the Canon 1.4x extender.
I haven't owned a 2x extender but I know two Chch based photographers that have and use them (on everything from a 70-200 F2.8, through a 100-400L, to a 400 F2.8L, and they both love their 1.4x, and almost grudgingly mount the 2.0x when there's no other way to get the shot. The 2.0x is a big compromise and I'd rather crop to get the result.
Personally I use my Canon 1.4x on my 70-200 F2.8L (Non-IS) and my 300 F4L and I NEVER have any doubts or concerns about the results.
3) Flashes
The Canon 430EX and 580EX (either the original or II versions) are both great... If you think you might be using multiple flashes at some stage, try and get the 580 first.
However, IF you are only going to use flashes for interesting off camera stuff with a manual flash trigger (like PW), then scour trademe for a non-EX flash, or even a non Canon flash. You only need a flash that has a manual power output adjustment (like a Canon 540EZ)... They are giving them away on trademe and do all you need for remote flashing (which I use heaps on my MTB stuff)
4) Wide lenses
Depends on the body, but I think 10d was mentioned... Biggest regret I have in selling equipment was that I sold the Sigma 15-30EX I used on my 10d years ago. It was sharp, fast to focus and hilariously wide.
I not have a 16-35 F2.8L, but I never use it at F2.8, and for MTB stuff or any landscapes I'm usually at F5.6 - F11, so the 15-30 would be been fine.
If you can find a Sigma 15-30 for under about $700 (they come up occasionally), grab it before I do.
Other options are the Wide Canon L's (17-35, 16-35, 17-40 F4L) and for a 1.6 crop camera, the Canon 10-22 EFs is just stunning. I don't know about the other brand ultrawide zooms.
Anyway - there are some hints from me - feel free to ask anything else..
Cheers -- N
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