Sounds like you are on the right track. I think if you can completely admin the box yourself the only downside is no spare hardware onsite if your box happens to die. If you rent a dedicated server you would assume the host would provide you spare hardware instantly if your system dies. Thats one benefit , but depends on your hosting company how warm and fuzzy that would feel
We will sometimes recommend the colo option to our new and old customers when they go too custom with the options that they need, or they simply are looking for ways to lower monthly cost. I think in the end if you are fairly sure you will do fine with a colo, and know you have a datacenter that will support you, the way you need it then it's a great way to go.
Stay away from desktops and towers. Most of the better datacenters will charge you more for the space in these bigger boxes. Get 1 or 2U servers when possible.
Good luck on your move