You can get the original Exile games for free here:
Blades did not get a second remake. Blades of Exile came with three scenarios and a program to make your own. The idea was that people would upload their scenarios to Spiderweb Software's website and you could play stuff other people made.
I've played through the first pre-made scenario, which was the shareware release. I have Blades of Avernum, which added one official Scenario, but I haven't spent much time with it and haven't played through the other three official scenarios. I guess I should get around to that.
I don't know if people still make new scenarios for it, but you can download old ones that people made online.
I have played through the second trilogy, which there is only one version of. It was made after the first Avernum trilogy (so the second version of those games, after Exile).
My rankings of the games, with some notes about what makes the different from each other. Part of the original concept behind the series was that players would have options about whether or not they wanted to do all the main quests (there are a million side quests so I doubt that many players completed them all). I'll say something about how that works out from game to game:
Avernum II: The Ruined World - This is the best loved game in the series, with the strongest and most interesting story. After the early part of the game you have alot of options as to what order you do stuff in, but I do feel like this is one where if you don't do all the main questions, you feel like you've just given up (partially a consequence of having a stronger story).
The only time since college that I've gone over 24 hours without sleeping was when I was finishing up this game (after only sleeping 4 hours the night before because I was up late playing it when I had work the next day).
Avernum: Escape From the Pit - The one has a well developed world but not much of a story, which makes it the most sandboxy game in the series. There are three primary quests, and you can do only one or two without feeling like you've "given up," so it's probably a good one to start with if you want to dip your toes in the series.
Avernum VI - The last game. There are some major choices that result in different endings, but you proceed through the game somewhat more linearly than you did in the original trilogy. This is by far my favorite game of the second trilogy. The story will have more resonance if you've played the earlier games.
Avernum III: The Ruined World - This has five major quests, but to finish the game you only need to do three of them (there are two sets of either/or/or both if you want, and one that's obligatory). It's physically the largest game, and one of the two games that's set in a different part of the games world, so the overworld isn't made of locations that overlap with most of the rest of the series.
Avernum V: The most linear game, but there are lots of choices where you have to do one quest or the other. I haven't replayed it but they obviously tried hard to make up for the linear progression with options that would give it more replay value. This one has a reputation for having alot of annoyingly hard "boss" type fights. It is the other game, besides III, that takes place in mostly different locations than the rest of the series.
Avernum IV: Eeeeh...it's okay, but I think this is by far the weakest of the series. If you play the newest version of the first trilogy then this is also the oldest game. I think it's too much of a rehash of the first three, and parts of it are a slog. That's not to say that I didn't enjoy it, but at this point I think this one is only for completists. There's nothing unique that makes me want to recommend it, it's just more Avernum if need that much Avernum in your life.
I think you can skip most of the major quests and still do the end game, but you'll get a more pessimistic ending.