OK, so each edition has its pro's and cons.
2nd and 3rd edition were really more about squad combat rather than army wide combat, so the rules can get bloated if you try to up scale the games.
4th edition was very gamy, and close combat armies could rick roll through you.
I would recommend 5th edition. The rules are not overly complicated, but some people are unhappy with the way close combat works in the charge phase. A quick fix is both players do d6 + I. There is still cheese, but its easy to spot now and stay away from. 5th also has a lot of extra material, different game modes, campaigns, etc
6th edition is 5th edition with some tweaks, more units and some very weird point values (like the wraithknight and scatter laser bikes) for some units, they also introduced allies, DON'T use allies if you want a decent game.
7th is where things went nuts, still playable, more tweaks over 6th, more consistent point value, but stay away from allies and formations, there is still obvious cheesy BS, like flying demon princes with 2+ rerollable saves that can body swap with another demon...
**I can't remember when the second release for IA 11 Doom of Mymeara came out, you WANT the 2nd release as corsairs are a legit army in it. They are not over powered, they were just decent. GW really screwed up corsairs after that though.
Rules, current vs older...
The "core" rules in 8/9 are straight forward, but has a TONE of "special" rules that have no uniformity. Then you have the complexity of strats and having to remember to use the right ones at the right time. Easier to learn the basics, how you move is far less important, it is far more important to use the right strat at the right time.
Older edition had more rules for the core mechanics. but had universal special rules. It takes a little more time to get the "basic" rules down, but its really not that hard to learn... Actual tactics matter, positioning matters, firing arcs. This to me makes it a better game as thing you control matter more and there is a big emphasis on risk vs reward, which also makes it more exciting for me.
It is fairly easy to get hold of older editions books, facebook has dedicated 2nd hands sales groups, some international some country specific. If you get desperate you can find digital versions of most books.
Power levels... are not balanced, if they were they use it for tourney's, its not a bad way to play though, it is easier to make lists, etc.
9th without updates... no, you really can't play it that way. Some codexes are super unbalanced, like dark eldar and Admech, you need the updates. Also I have tried to play it with out strats, it is more fun, but it hurts some armies alot.
Read a campaign book from a previous edition and then look at 8th/9ths version of it, they are basically pamphlets once you take out all the reprinted lore and rules. So yeah, they technically have it, just don't expect much from them.
So, my group is older, we tend to play the older editions, not out of familiarity, we give the editions a fair go, and settled on 5th. We do see other people playing the new editions having dummy spits, generally when they are playing a random person in store. When you can see how easy it is to apply over whelming fire power, we can't really blame them, there really is something that doesn't feel fair about the game. For me I think the lack of risk vs reward is where it comes from, in older editions if you wanted to do something powerful it usually had a risk of going wrong, like deep striking.
While the new editions are not the right fit for me, I would still encourage you to play both the newer editions and at least 5th ed so you have something to compare it to.