No offence mate, but 5mbps for $150? Not going to happen! Thats $30 / mbps which for such a small committment is extremely unlikely (Thats cost price on Cogent!!!). You need to raise your budget a little, double it and you'll be closer to the mark.
Why do you want to run Windows if the primary purpose of your box will be as an FTP server? Windows is expensive! :-)
I suspect you could more easily find an inexpensive host for what you need by simply purchasing a virtual/shared HTTP/FTP hosting service.
Realistically, $150/mo for 5Mb is going to be tough, especially when you include a host's cost on purchasing or leasing a $1000 server in that!
If you look for a per-GB billed virtual plan, you may be able to get what you're really after without dealing with the hassle of administering your own server, and worrying about 95th percentile pain at the end of the month!
What kind of total transfer traffic do you expect for your site in a month, in GBytes? Don't believe anyone who says 1Mbit/sec 95th percentile = 320GB/mo, as typically your bursts will throw that figure way off.