Question regarding SearchAPI and Index performance/scaling
Regarding Search, I would suggest to go with external Search System, like ApacheSolr, Elastic Search, Algolia, etc. This will make sure that database will not be overloaded with content index.

I'm working on a project where we currently have almost 2 Lac nodes and around 30-500 nodes created every day. The number of concurrent users are 1-20 max. We're using ApacheSolr (Solr hosted on different server) with Search API and Search API Solr Module and results are retrieved in less than 1-2 Seconds (The Project is internal of Client's Company so can't share the link here).


  I was thinking of just using straight views, and trying to do search
  filters, but it's not scalable and after a few months to a year it
  would be quickly unmanageable.


Using view for rendering search results will not help much in terms of instant results. Preferred way will be writing custom code to query based on keyword and show results based on requirement.


  Alternatively, thinking out loud, could I have it submit the new node
  to the index on every new node addition?


Indexing of Node after Creation/Updates are managed by module. The search indexing is normally done when running Cron. So setting up your site Cron to run every minute will make sure that the nodes are indexed instantly.

Hope this helps.