The Canon MPE 65mm macro lens isn't just a macro lens, its only a macro lens. The lens itself cannot focus on anything further than around 6cms or so in front of the lens and goes from the magnifiaction of 1:1 (the maximum that regular macro lenses achieve) down to 5:1 (5 times life size, which is bringing you well into the magnifications for springtails and the like).
It's also one of the hardest lenses to learn to use, let alone use well and its not the kind of lens I recommend to beginners unless they have a very specific requirement that simply can't be done otherwise (and also a clear indication that, not matter the difficulty, they will work on getting the shots). It's one of those lenses that is rare and even in macro circles its not the most commonly owned lens.
As a starting point I'd suggest a regular macro lens, Canon, Sigma, Tokina, Tramon - any brand will do (optically they are all on an even footing with regard to macro photography) so its a case of fitting your needs and criteria and budget against what they offer you. These regular macro lenses will retain focus from infinity all the way to the 1:1 magnification. You can also use attachments to increase this magnification (normally at a loss of infinity focus) should you wish to experiment with higher magnifications.
Examples of lenses you can consider:
Canon 100mm f2.8 macro - 100mm f2.8 IS L macro - 180mm f3.5 L macro (note the canon 50mm macro is not a true macro lens unless used with its lifesize converter - at which point you've spent more than most all in one macro lenses so is generally not recommended.
Canon EFS 60mm 2.8 macro (EFS means it only works on the 7D and other crop sensors and won't fit fullframe bodies like the 5D)
Sigma 50mm f2.8 macro, 70mm f2.8 macro, 105mm f2.8 macro (OS edition now released), Sigma 150mm f2.8 OS macro (original version without OS discontinued but still on the market second hand and worth considering), Sigma 180mm f3.5 macro (discontinued but still top rate and in the second hand market)
Tokina 100mm f2.8 macro
Tamron 60mm f2 macro; 105mm f2.8 macro; 180mm f3.5 macro
Note some of the 3rd party options are crop sensor only as well, but I'm not sure on the specifics of all of them. Each of those lenses would give you 1:1 magnification with quality results; however they'll each have their own differences such as different focusing systems; compatibilities with teleconverters; internal focusing etc... So its good to read up on them adn compare and contrast what they are each offering you.
For insects 90mm or longer is the advised starting point just to give you more distance between the subject and the camera when at the 1:1 distance to lessen the chances of spooking the subjects. Note that the framing at 1:1 will be the same for all the listed cameras, what the focal length changes for macro is the distances, where longer focal length lenses give more distance between camera and subject as compared to shorter focal length macro lenses.
Also note that one can easily use a regular macro lens and the MPE alongside each other, the lack of greater than 1:1 magnification on the MPE means that for whole photos of larger insects like butterflies, dragonflies and the like is impossible, so a regular macro lens retains its use.
My all means if you feel that you need the magnification of the MPE go for it, but if not and then I'd work up toward it. Its not just focusing but also lighting that gets more critical with the MPE, and as stated you'll want to keep part of your budget for lighting.
Lighting wise you don't have to go overboard with complex macro setups - a single speedlite flash (such as a 430EXII or 580EXII) combined with a softbox (Lumiquest original softbox or homemade) and a bracket (RAM mount is the best I've come across - more details in this link here:
My very flexible Macro flashbracket: Studio & Lighting Technique Forum: Digital Photography Review
That can provide more than enough good quality lighting for macro from 1:1 all the way to 5:1. Furthermore I encourage people to get a regular speedlite (like those listed above) as a first flash because, unlike the weaker, more specific macro flashes, a good speedlite unit retains its uses not just for macro photography, but for pretty much any field of interest with photography. Giving you a device that will have a lot of potential use for you and a good starting point for flash controlled/aided lighting.
Just to finish I'll give you a few examples to bring 1:1 and 5:1 magnifications more into the real world with what you can expect. These are taken on a crop sensor DSLR which, whilst the magnification remains the same, due to the nature of crop sensor if you print out from crop and fullframe next to each other the crops appear to have more magnification.
1:1 of a hoverfly:
1:1 of a UK 2p coin
3:1 of the same coin
5:1 of the same coin
As a final point regarding fullframe and crop sensor this is really an iffy area - neither one is the "best" choice and each has its advantages and downsides. I'd advise considering your uses outside of macro and also testing out the two cameras in a shop to get a feel for their differences in how they perform (since real world testing is the only way to get a real idea to yourself of the differences). Note you might well not find many shops with the MPE to test and few willing to order it in just to test.