There are a few conferences dedicated to ecommerce search. Mices is pretty good. I did not go there this year but I know some of the people behind it. Good community and lots of stuff happening.
Two points here.
- 25 million is really not a lot for most search engines. Something like Elasticsearch can easily deal with that if you deal with it properly. And there are plenty of equally capable solutions. I have worked with logging clusters that processes log entries by those numbers on a daily basis. A modestly sized cluster goes a long way for that. Bare metal is cheaper than cloud for this. But a couple of simple servers with decent CPUs and memory and SSDs should go a long way here. Start worrying once you hit a few hundred GB of storage used. Anything below that is easy to deal with.
- The key challenge with this volume is not performance but search quality. Building a competitive search engine is hard. You might have thousands of potential matches out of millions for any given query and your job is to pick the best 3, 5, 10 (whatever fits on your screen) ones. This is hard.
So, what makes for a good answer is the key question to answer. All the naive solutions for this problem put you at the bottom of the market in terms of competitiveness. If you can't do better, you are just another low quality search engine not quite solving the problem. The bar is high these days for a good search engine and most of the better ecommerce companies have highly skilled search teams working on this.