With non-native integrations you'll need to use middleware. The basic idea is, Marketo plugs field mapping to the middleware, Netsuite plugs in field mapping to the middleware and the middleware crisscrosses fields to the correct field in the other system.
All integrations are done through SOAP or REST and will require API calls, so if you already have a solution using API calls intensively, take this into account. The shorter the sync cycle, the more API calls required. The number of API calls can also be affected by how efficient the sync is. Obviously the more fields/objects you have synced, the more API calls required.
Managing the field mapping can be tedious, the initial set up requires you to map all your fields and objects in the middleware. Ongoing, you'll need to create custom fields in both platforms and then map them in the middleware. Depending on the smarts of the middleware you've chosen, this can be less tedious.
Getting a bit more technical, related to how efficient some middleware syncs, some middleware keep metadata to ensure and each field has their own last updated date, this allows only 1 field to get updated in a sync cycle rather than the whole lead record. Also a technical issue is how each system manages duplicates and the middleware will need to be configured to work with both accordingly.
Overall, if you execute well and already have a clean database, it shouldn't be a hard project. Expect it to take a few months to get up and running...
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