This is not the sort of situation I would invest in. If you are billing by the hour, it is unlikely to be a leveraged business, and the investment return is unlikely to be as comfortable. Additionally, due to the smaller size, there would have to be many, many investments; it's hard to find many of very good quality.

It's very hard to sell to large sized enterprise without supplying custom work. You're right that the custom work isn't leveraged, but it's a prerequisite to get your product installed into their (often silly) IT structure.

This only requires a portion of your engineering and support resources, and there is no reason you could not establish strong ties to an existing consulting organization to be the "consulting services" arm of your company.

That tends to be the cash-cow that enterprise software vendors grow into (SAP, Oracle, et al), though I agree that it's not the place you want to target from the get-go. It probably does have some seductiveness if you're boot-strapping.