Hello, my name is George Bouris.
If you check the source code of the page for the advanced search, you will easily locate the "searchForm" and, inside this the "lotID" text-input element.
So, I'll use a template form, where I'll insert the value of the needed lotID (taken from the excel or csv file - my suggestion is for csv) and submit it. I'm not sure if the message "No Item(s) were found. Please try again!" returned in both cases: when a not existent lotID entered or when it's just out of stock. Anyway either locating this message or what it returns when lotID is temporary unavailable, I'll detect the situation that you're asking for.
Answers to your questions
1. I'll write it in C#. In my opinion this is the easiest approach, both for the development of this utility and its maintenance.
2. The main objective of this project is for you to be updated via email, when a lotID is not available.
3. I think I have answered this question, already - at least for its core functionality. I assure you that I totally understand the need to send an email, when a lotID returns no results and all the details of the configuration and the rules for the repetition of the check.
The only thing that I'd suggest to be done differently is the input and presentation of the configuration parameters. In my opinion they should be contained in the main form, so you can easily have the complete picture of the tool. Anyway, I can do it either way.
Thanks