#1 – How does it work?
You’ll probably get as many different answers to this as there are vendors – but you may not get the whole truth. Bear in mind that a lot of search engines share what theoretical ideas they apply. An engine might use a vector-space or probabilistic models for ordering results, for example. Most will create an inverted index.
#2 – How fast is it?
Every search engine will take a finite amount of time to index a document or produce search results. Some of these processes will be limited by how fast data can be written to or read from disk, some by how fast the processor can do calculations. The key point is whether this time is going to work for you – will your users care if some complicated queries take ten seconds rather then a fraction of a second? Is there a time in the middle of the night when the system can spend a couple of hours building a new index? Watch out for silly answers such as “it’s instantaneous”.
#3 – How does it scale?
Whatever data you have today, you’ll have more tomorrow! How many servers will you need today, and how easy is it to add more in the future as necessary? Will this affect the speed of indexing or searching? Cloud-based solutions can help, especially when the amount of data or queries can be variable.
#4 – How much does it cost?
This is a question with several potential answers: the cost of a software license (of course, with open source code this can be zero), the cost of integration and customisation so the engine fits your requirements and the cost of ongoing support. Beware of a solution that promises much, but only after months of customisation. You should also ask how the cost scales with any growth in the number of source documents or users.
#5 – What happens if the vendor is taken over or disappears?
If the vendor is acquired by another company, or goes out of business, what happens to the software? The new owners may force you to move to their preferred solution, or in the worst case you can be left with no support for an obsolescent product. Ask if the vendor offers escrow. Open source licensing may also be a solution.
The above is not meant to be a complete list – feel free to suggest further questions!