First off, what version of the game are you looking for? There are many.
If you are planning to run a server you'll need to be running some type of zone guard to patch the exploits that exist
(item duping, invisible characters, crashing the server).
Those only work with the zone server they were designed for... so you will need to ask the vendor what exact files they are using. If you aren't running a server and just want to develop content for the game you can use any.
Generally people use 2.2.3.2 (Golden Age). These are the files I put together whilst with RF-Dev
Alyx — 2021
RF-Dev 2.2.3.2 Client Size: 1224451822 bytes (1.13 GB)
SHA-1: C14311154FD18B92A71087DD773FE319D1B6FB55
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RF-Dev 2.2.3.2 Server Size: 242391498 bytes (231 MB)
SHA-1: A15099A44D0EFFF267EFF502BBC13EA163B194D2
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And the guide that goes with them.
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