There's always discussion here as to which is better - a separate domain, a subdomain, or a folder of the main domain. In general, the last may be best, particularly if you want to share login info. OTOH, a subdomain may make splitting the servers slightly more straightforward if that's ever required.

Moderation probably won't be a big deal right away. As traffic develops, you can try to cultivate volunteers from the community. If the site is very commercial, you can try to draft employees, hire part time mods, or contract with a service firm.