[What would be the best eAthena version to pair with this old 2004 client?]

You can extract recvpackets.txt from an unknown Ragexe.exe to analyze.
The packet list is a hint to the version.

Each packet is in the emulator.

eathena packets
cronus packets
rathena packets

[What would be the best eAthena version to pair with this old 2004 client?]

The recvpackets.txt is 2004-08-16aSakexe as we see in packet_ver: 9

Probably any rev >= svn540

[SAKRAY 2003?]

2003 FreeRO client
untested - use at your own risk
https://web.archive.org/web/20030806063115/http://izlude.com:80/fRO_0725.exe
https://web.archive.org/web/20031021173933/http://www.izlude.com:80/fROclientInstall.exe
https://web.archive.org/web/20040114142234/http://hkwonton.izlude.com:80/Client/Patch%203.2/fROclientInstall3.2.exe

[Req> 2010-07-30 Full Client]

Found this RAGRAY 2010-07-30
Seems to be ok.
A file joiner will be required.

Spoiler

[RAGRAY-FULL-2010-07-30]
https://www.mediafire.com/file/nf3qw6184oqqwkz/RAGRAY-FULL-2010-07-30.exe.001/file
https://www.mediafire.com/file/1lw4l88fb1c6nn0/RAGRAY-FULL-2010-07-30.exe.002/file
https://www.mediafire.com/file/96ixlm736vkk4kj/RAGRAY-FULL-2010-07-30.exe.003/file
https://www.mediafire.com/file/35u7tljg2wzgrr1/RAGRAY-FULL-2010-07-30.exe.004/file
https://www.mediafire.com/file/9halv29f8hejq15/RAGRAY-FULL-2010-07-30.exe.005/file
https://www.mediafire.com/file/pan79w5izap1qkp/RAGRAY-FULL-2010-07-30.exe.006/file
https://www.mediafire.com/file/a5e8v8cc5ykt172/RAGRAY-FULL-2010-07-30.exe.007/file
https://www.mediafire.com/file/o1l2col9v1t1oba/RAGRAY-FULL-2010-07-30.exe.008/file
https://www.mediafire.com/file/prvek1vnes2ilz6/RAGRAY-FULL-2010-07-30.exe.009/file
https://www.mediafire.com/file/o10f52zrej0i92b/RAGRAY-FULL-2010-07-30.exe.010/file

mediafire password: arkandia

[file joiner]
https://www.igorware.com/file-joiner/download

[source]
https://web.archive.org/web/20120415234236/http://winuxfunboy.blogspot.com/2011/11/update-3ceam-606-ragnarok-offline-3rd.html
 

 

[Need very old Client]

untested

 

Spoiler

https://www.4shared.com/rar/CIrHlhUY/2007-02-20a.html
https://web.archive.org/web/20100414231221/http://svn.games-service.net/
https://web.archive.org/web/20070319152817/http://www.dzerox.com/RO/Tools/
https://web.archive.org/web/20060112012939/http://svn.dac-ro.net/
https://web.archive.org/web/20040711125401/http://eamirror.skamfroj.net/
https://web.archive.org/web/20070214064500/http://www.taicomjp.info:80/forum/viewtopic.php?t=76