Tassie,
You are already on the right track by allowing only those who do design through you to also get the hosting. I would never suggest for you to do anything else. The other part is that major hosting companies offer great deals, but lock people in to two year deals - you want to stay away from locking people in. I would offer only year by year, that way if you ever decide to get out of hosting you have no further obligation to them. This, however, may make them worry (the possibility of you deciding you do not want to do it) this is why I always add, "if something were to happen and I got out of the hosting business, I would help facilitate your move to another hosting provider" (and you should always have a backup hosting provider that you can send them to).
You should always prepare a contract, nothing fancy, the same sort of contracts you would use in your development business. You should always get your hosting money upfront. Never go month to month, that is a headache you do not want (although, I can see do a six month deal, if someone really, really needed it.)
Also, If you ever do hosting make sure you get on a VPS (virtual private server), its like your own server so that you don't have to share bandwidth and you don't get charged extra by your hosting provider. Most hosting providers know if you are going to resell their services, so they will oftentimes suggest that. The cost is much higher than what you will be used to, but it usually only takes a couple clients for you to start turning over some profit.
I will add more later, if I think of anything, and I will watch this post in case you have any other questions.