Single server should be fine, as long as you take precautions. I would future-proof by getting a quality RAID controller that supports SAS-2 disks (6Gbit/sec) instead of SAS which has 3Gbit/sec throughput. Get an enclosure with plenty of slots, so you can add additional RAID1 or RAID10 devices in future and balance the load. Exchange 2010 is not a big CPU pig, and most of the time people are I/O bound, so I would initially get a single quad, and spend the money I save on better I/O.
I would also buy a small SSD, use it for the boot device. This will make a big difference because all of those small 4Kb I/Os that have to get done by the host O/S won't be chewing up precious IOPs that are on the exchange database devices.