What you ask for is actually pretty complex task.
I've covered this in my answer to this question on Stack Overflow:
How to detect that a file is being uploaded over FTP

Actually I was hoping someone already had a script written since I think it's a fairly generic task. I've been tasked with figuring it out though I haven't any WinSCP experience.

That's what -delete does:

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Though this won't cover the 1. It is rather vague requirement anyway. Not sure what you exactly mean by it.

I'm having trouble with what should probably be a simple operation.
Background.
I have a server(A) generating logs behind a data diode. The logs are sent through the data diode to another local server(B) which will FTP any received logs to a remote FTP server. Once FTP transfer is completed then server(B) will delete the transmitted log file locally.
The script will be run via Windows Scheduled Tasks every 10 min.

1. Check for new complete files (no partials) in local folder, if none then end script.
   
2. If new files exist then FTP all new files to remote server directory.
   
3. Delete newly uploaded files in local directory then end script.
   

Not sure how to avoid deleting any files that were arriving as or after script was started.
New to WinSCP & scripting but it seems to be a fairly common task. Maybe there is some premade subscripts I could piece together into a new script.