@Sandra, I'd roll this back even further and wonder if you had an experienced Marketo developer recommend this architecture.
As an experienced Marketo Forms 2.0 developer I have to say that using the server-to-server SOAP API (which is itself deprecated in favor of REST) instead of the client-only Forms API is a very strong choice that should only be made by someone who understands all the options.
Generally, there's no reason to choose any of the server-to-server APIs unless [a] you must persist saved data on your server as well as Marketo's (even then, there are client-side workarounds, assuming you're storing all the fields in both places) and/or [b] you must augment the Marketo fields with data that is only available on the local server. I don't see either of these requirements in your brief spec (not that they don't exist, but they aren't presented).
Listing the full technical differences among the options is not appropriate for this forum, but I'll note for now that using the SOAP API limits you to 10,000 form posts per day. That may sound like a lot, but a hacker can easily quench that 10,000-call limit in a couple of minutes. And even if you optimistically assume all form posts are legit, that 10,000-call limit applies to all applications using the SOAP API, so any other integrations are sucking away at the limit as well. In general I have been telling people that the SOAP and REST APIs, while powerful and indispensable, are suited for batch applications, not for the one-to-one form proxy pattern it sounds like you are describing.
I hope you are open to reevaluating your architecture because a Marketo developer worth his/her salt (i.e. the candidate you want!) would want to know how you got to this point, whether or not they come to the same conclusion. Good luck!
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