I started with a reseller many years ago, we now run servers rather than resellers, however we do have reseller customers.
In general a reseller wants a stable non-oversold server with good uptime (just like a shared hosting customer would expect). Support is not too critical as the reseller provides the customer support, however you need to know that in the event of a system problem the host will be on the case quickly AND give you a useful update as to what the problem is and when the service will be back.
If you value your business, don't be tempted to go for an unlimited or the world for $1 type of reseller packages that people are now offering. Your host may let you down, but in your customer's eyes, it's your service that is down and they're generally not bothered about your issues with the host as you have their money.
Look for a "proper" company that's registered with the necessary authorities (in the UK, that would be Companies House and probably the VAT man). Anyone not publishing these details (or is not prepared to give them on request) or only has an email / MSN for their contact details is most probably not legally registered as a business. Do you want to risk your business with someone who's potentially trading illegally?
How you break the resources down to resell has to be down to you, as Matthew (above) points out, you shouldn't try and compete with the long established players (or even the kiddie hosts). In my experience those who shop on price alone will not be with you long and will probably cost you a lot in support time.
Most people don't need Gb's of disk space, so offer realistic packages at an honest price and you'll get some good & reliable customers.
Also, don't expect to become rich overnight
