I wanted to know if there are specific account levels for Marketo. Are there accounts used by customers not having access to REST APIs ? Accounts with admin level access can only use Rest APIs ?
Wanted to also know if webhooks support is there for all customers ? or even this is specific to particular levels of customers ?
Spark accounts don't have API access. I don't know what you mean by "admin-level access" exactly; REST API access works via proxy API-only users, which don't log into the Marketo UI at all and might have only read-only access to a single type of record.
I don't think Spark supports webhooks, either, in answer to your other question.
That said, you do need an admin-level account to implement an API integration, if for no other reason, to create the API "user", create a launchpoint service for the external application to call, and access the client ID, client secret, and authenticated user to successfully execute API queries.
http://developers.marketo.com/ can give you most of the information you'll need for API integrations.