You do understand that Colo, means that you build a server, and pay for most of its software and send it to them.
You would be best off renting/leasing a server, in all reality it is much cheeper and makes upgrades alot easier in most resepcts.
For OS choice, what do you work with most? Linux or windows, how much expirence do you have administering with either of them?
I would suggest a control panel software if you are not familiar with hosting itself, and if you decide linux and to rent from theplanet i would go with cpanel / RHE.
A sleek interface is nice, but its the actual opteration of the backend of the system that makes it worthwhile.
Plesk to me seems to be very cumbersom and you need to really go out of your way to do things.
Cpanel is only expensive if you buy the license outright, if you rent a server from theplanet it only costs 25 a month, which is signicantly less than buying a year license or owned license from cpanel itself :)
I would really recomend renting a server from theplanet, if you are going to host get a nice server, no celerons, nothing less than 1GB of memory in the server either.