If you’re interested in Client Modification; I’d recommend lcdr’s Unpacked Client. However, if you’re interested in just a working Client for gameplay; If you get into the Alpha, just wait for the instructions that lcdr will send. Although, I’d recommend this Client if you just need one.
I’ve personally never had issues with any clients not working, and so long as all the required information exists in a client, there should never be an issue communicating with a server. However, with stuff such as small modifications made by lcdr, I’d be very wary of trying to use a client different from his own (although, as he’s said, his version is effectively the same as the public version, so there’re very unlikely to be any game-breaking modifications to it!)