Oldhammer is focused on the first 2 editions of 40K. There is a rather dramatic shift from 2nd edition to 3rd and later. I believe some people have adopted the term "middlehammer" for OOP editions of 40K that don't fall under the Oldhammer umbrella. As such, I'm afraid this site won't be much help I'm afraid. Many members will be unfamiliar with editions 3-5, or actively dislike them and having abandoned them for Oldhammer 40K. Even those who are familiar with edition 3-5, and are fond of it, would be unlikely to discuss it much. Not, so as you understand, as a snub to yourself, or these editions, but out of respect for the rest of the forum. It has been built up and maintained by Oldhammer fans, the more time and effort spent of other editions, the less focus is on their favoured editions. It would be rude to drown out Oldhammer stuff with other editions. Perhaps there is a middlehammer group of forum out there that could be more help.
That said, since I think it would be rude just to dismiss your question.
Editions 3/4/5 are pretty close, far, far closer than 1st and 2nd edition of either of those editions to 3rd. Mixing and matching the codexes should mostly work. Indeed many factions were left with older codexes as the editions changed. It is an out of print product, so feel free to pick and chose the elements from each that work for your group. My memories are vague, but I think that 5th ed missions were better balanced for late game comebacks, the effectiveness of vehicles, especially transports varied wildly. If you go with the very effective 3rd edition ones, I recommend doing away with overcharged engines. I think 4th edition overcompensated and made them death traps. Find the balance that works for you guys, don't stick religiously to one, as not even GW found the balance they liked, just make sure everyone is on the same page. Most importantly spend time playing, enjoying the game and then deciding what works, don't pore over the books like legal texts.
Useful books include any of the codexes, they should be cheap on ebay. Again, try and focus on fun lists, there aren't any tourney's for 3/4/5 there isn't much point in over competitiveness.
The Chapter Approved books for 3rd edition include alternative army lists, new assault and transport vehicle rules, clarifications and a Vehicle Design System useful for adding stats for other vehicle models.
Right, when it comes to Chaos in 3rd edition there were 2 codexes/ The first was pants. The second had much, much more choice and character. More potential to make broken lists too, but like I said, I imagine you will be playing for fun rather than competition in an OOP edition. So simply avoid the brokenness.
The witchhunter and demonhunted codexes include adversary rules, for cultists, mutants, rogue psykers and so on. Use these to make stuff like Genestealer Cults or others.
Codex Armageddon and Codex Eye of Terror include fun little lists as well. I think Codex Armageddon Salamanders are my favourite Salamanders.
What you
need is just the big 3rd edition book, it already has lists. The rest is just more options. I think I'd probably steer towards 5th edition rules and missions with mainly 3rd edition Codexes and take advantage of the Vehicle Design Rules to add new vehicles and reskin Adversary army lists to make Genestealer Cults.
But most importantly, play. Build terrain, paint armies, play and tweak, play and tweak. Enjoy it, just maybe don't expect a lot of input from this forum
