Most bang for the buck - find a used Canon 550 EX which can be used as a master or a "slave" or just an on-camera flash. It will remotely control any EX flash. The older 420 EX flashes are great "slaves" and will work with any Canon "master" e-ttl flash system.
The newer 430 Ex and EX II can be used in both manual mode and in ettl mode (as "slaves") while the 420 can only be used in the e-ttl mode, but e-ttl is pretty foolproof...I use a 550 ex and three 420s and it cost me less than half (maybe less than a third) of what it would have cost me for new 580 and 430 ex II units and they haven't been anything but perfect - I use constant flash in the studio, but they are expensive and not portable and need a lot more work to get right (the E-TTL system will meter the flash for you).
I was a grip, a key grip and eventually a lighting director in the motion picture industy (as well as some TV and live theater) for over 40 years - so lighting is easy for me, but using manual flash is something I never used in all that time, so having the e-ttl system is great for me....it's just always right.
The newer aps-c cameras like the 7D and the 60D can trigger the Canon flashes (any ettl flash that can be used as a "slave") with the pop-up flash (it doesn't need to actually flash when the shutter is open - but can be used either way).
Radio flash controls have a much greater range, but I don't really see much need for using flash at 100 yards - or in fact outdoors for much of anything other than fill flash in sunlight. (where an on-camera flash is usually adequate - especially the larger 5xx series) -
From my experience, when possible, I'd prefer to use reflectors in the sun, but it is far easier with an assistant - where a flash doesn't require an assistant.
The newer stuff is of course more feature laden, but I work fast and I like using the ettl mode ...I can't understand why anyone would pay for it and not use it???
The newer Ettl II flashes take distance into consideration more than the older version 1 stuff, but if you get into using multiple flash systems, you'll figure out if you need to move a flash closer or further from your subject - use a different diffuser, etc..(umbrellas are cheap and bouncing flashes off ceilings and walls is free and where ettl shines)...you'll get perfect exposures whether you use direct flash or bounce of a ceiling or a wall to the side or behind you - the only difference is how evenly you want your lighting. Pefectly even lighting makes for boring photos - a great reason for learning to light in black and white as I did in film school.
Best of luck,