Personally, I have a massive distrust of wireless stuff. However, it's a real pain in the ricker that those two buildings in-between won't co-operate. It's hardly like you're fitting massive air-conditioning ducts between the buildings, is it?
In the same way, we're in a row of 4 buildings, but we own all four and communicate between them using 100mbps fibre. As it turned out, fibre was a major blessing in disguise as the amount of machinery, plant equipment and associated power circuits we have onsite has increased proportionally. I think as it stands, even 1gbps copper would be saturated with emissions from this equipment - even if we used STP cable.
Site surveys unfortunately are par-for-the-course in terms of wireless setups. And even after that, you can end up with bizarre connection issues (like 3Com wireless routers not liking certain Intel wireless NICs in laptops - as we've found out!). You could always try infra-red links? Not sure of the costs, but for that type of distance, IP traffic won't be a problem via infra-red. A smallish IR mast and receiver box on each building, simply pointed at each other. Like most non-wired kit though - they need serious security at both ends. There's also radio as an option - but for the distance involved, it's probably total overkill. Some of the high-spec radio IP kit has a range of 35+ miles.
What is the approximate distance between the two buildings?
D.