The 7d is a very nice camera, but you'll want to be careful of the lineup of L series lenses with that body. Many of them were really intended for full frame cameras, like the 5d line up. They are almost a waste on dx camera bodies like the 7d because you will have light drop off and vignetting, meaning you will have to crop away the outside portion of the photos anyways. Thats' not to say you can't use them, it just means ask those kinds of questions form someone who knows canons really well. I could give you the right info on the nikon series since that's what's in my bag.
You can get reasonable pics with the kit lens so long as you provide lots of light. But overall, the L series are the cat's meow. The problem as you know is the value. Because they are made for pros who make a living from photography, paying 1.4K for a lens is really a drop in a bucket for those guys.
Do not discount the micro lenses. Yes, they are meant for shooting really small stuff, but they also make great portrait lens from 8 feet away. Lots of my aro pics were actually taken with a nikon 105mm f/2.8 lens, which is considered the best micro lens on the market, bar none. They are also benefits to doing it this way. You can get some wicked depth of field from shooting further away from the tank. Oops, I just let out one of my "tricks of the trade".
I will go on record as saying that the canon 60mm f/2.8 will be just about the best lens you can use in aquatic photography as well as general use. It is very similar to nikon's 60mm. I have that lens in my bag and it totally kicks azz.
What I'm going to suggest you do is go to a genuine photography store and start handling the different bodies you're interested in. Ask about the L series of lenses being used on a dx body, like the 7d. Take your time making this purchase as you will be investing a considerable amount of cash. Once you know what you want, then go looking online for it where the prices are.
Personally, I don't buy online because I deal with Dons Photo. They pretty much match most prices and my customer loyalty has earned me several very good discounts.
I cannot emphasize enough that you should know what you want before you buy it online. Returning camera gear purchased online can be a royal pita. Try not to buy anything from an online store unless they have what you want IN STOCK. If you let them back order your gear, you could be waiting up to 6 months for it, or they might try to pawn off some 3rd party offshore crap on you. Be a smart shopper.
Flashes? If you're really only intending on doing mostly fish photography, a single canon 530 or 430 will do the trick for most uses, including shooting pics of your kids or pets. The thing with canon and their wireless flash system is you need either a transmitter or a cable that basically tethers your flash to your camera. Most cables are only 3 feet long and are rather expensive. Of course I'm hoping you understand that I'm talking about using flash over the tank? This is where Nikon holds an edge. Nikon's wireless system is built into most cameras now and is referred to as "Commander mode". You can get a transmitter like the one I use, but you can fire the nikon flashes wirelessly by simply triggering the flash on a nikon body. They don't all have commander mode, but the higher end models all do.
Canons and Nikons are so similar these days, it really boils down to how the camera feels in your hands. I personally thought the canon bodies felt awkward. That plus commander mode helped me to decide to go with Nikon. Canon has a better line up of lenses, but unless you're planing on spending thousands of dollars on lenses, both companies provide what you need.
I will say that most high end gear retain very good value if you take care of the gear. It's not unusual to have a very good lens for several years and then sell it used for 15% less than what you actually paid for it. They are for the most part built like tanks. Remember, most of this stuff is made for professional use. Day in and day out, removing lenses, lots and lots of shutter actuations and vigorous use when in the field.
As for lenses, try them in the camera store. Really give the 60mm a good try. It will do close ups and it makes an excellent portrait lens. It will also be half the price of that 24-70mm. Zooms are nice, but the fact that it zooms puts it at a disadvantage, imo. It's also meant for full frame cameras, which means that with the 7d, the sensor will not be able to use the lens to it's full advantage.
You're much better off with prime lenses than with zooms. I'd rather have 3 good prime lenses like say a 35mm, 60mm and 100mm than an expensive zoom. Primes simply take better pictures, have less distortion and don't have soft spots like the zooms do. It's inherent since zooms are basically compromised lenses.
God, there's so much info and so many ways to go, lol.
ps-most non-oem lenses are crap. I have tried most sigmas and they are simply not worth it. Tokina makes a wickedly excellent lens in the 11-16 f/2.8 but that's pretty much it and Tamron? Well, let's not even discuss Tamron.
Check out dpreview.com and kenrockwell.com for some good factual reviews on camera bodies and lenses.
