Do you have enterprise-class clients? If so, ask them what kind of SLA they are looking for.
Usually, startups like yourself don't really get the kind of high-grade, industrial-strength, enterprise-y clients that care about SLAs. Honestly, as a startup, you can safely ignore this issue until the first time you're doing a deal for more than (say) $50,000 and you're negotiating a custom contract and the client really wants an SLA.
Personally, I'm not a huge fan of SLAs, as a client. When you study them you discover that they're not worth much. The services have big outages anyway, because "shit happens," no matter what the SLA says, and then they credit $3.24 to your account because the SLA never really gets you much, and you're supposed to be happy.