If you want to save on startup costs you may be able to lease
CF from your webhost - many offer fairly cheap monthly rates. Of
course, purchasing the license outright saves you money in the long
run. When you're leasing though, the webhost will often upgrade the
CF version for free when a new version comes out and continue to
charge you the same monthly rate. Also, they're sometimes more
likely to help out with config issues if you're using their license
rather than your own.
Also, on the topic of Standard vs Enterprise, running
multiple instances of CF in the Enterprise environment may be
preferred because then if one instance crashes it doesn't affect
the other - so your sites are a little more independent, rather
than CF being a crutch that could bring them all down.