If I understand correctly you want to start selling reseller hosting? If this is the case I would suggest you start with a VPS at a company with whom you can easily upgrade your VPS as and when you need it.
Things to look for:
SLA - service level agreement, read it carefully, so you don't get stung.
Location - If your clients are predominantly US, get a US server, if Europe, go for UK/France/Germany, etc.
Hardware - Don't get stuck on an aging dual core with no growth potential for yourself. Find someone using quad core, plenty of ram, and check they have RAID (preferably hardware) so if the worst happens and a drive fails, your data will be ok.
IPs - As a reseller you will need a fair few IPs, check your host will provide them, what they cost, how quickly they provision them.
Support - If you aren't an experienced admin, go for managed if you can afford it, otherwise unmanaged will give you a lot better deal but you will be largely on your own.
Lastly, as always, look for a host offering realistic specs not 'unlimited everything' as you will quickly find your on an oversold, overworked, under-performing server.