What type of climate are you operating in? I'm in Canada, thus putting any equipment outdoors is a big no-no, unless you want to fill it w/ antifreeze.
Assuming you're in a more temperate climate, I imagine the biggest issues you'll run into are weather-resistance. The expansion tank isn't as much of an issue, as you can spec it easily enough w/ a weather-proof casing. I'm not so sure on the pump, although I can guarantee you that it will cost more to provide a weatherproof pump than an indoor equivalent.
The only thing that really matters is that you pump away from the expansion tank. Place the expansion tank on the outlet from the boiler, as that will be your point of highest temperature, and thus the point of greatest water expansion.