If I buy the WM expansion on GOG will it work on my PoE I bought on Steam?

No. If I'm not mistaken you can't even buy it.

That said, the Definitive/Hero Edition is only slightly more expensive. Or wait for the upcoming summer sale on Steam.

Personally, if it's your first time playing PoE1, I would recommend playing the original campaign and after it adding the expansion. It's true that WM doesn't impact directly the campaign, but the experience of surviving Endless Path and your lvl progression changes forever once you have the expansion. Also, the dynamic of the soulbound items also changes the strategies of what to visit first.

Thank you for your input.

This is why I asked the question. For example, Baldur's Gate 1 provides a different experience without the expansion. True, the new areas are separate, but the extra XP and items you get from them, makes the orginal campaign different in some ways.

Baldur's Gate 2 on the other hand doesn't suffer from it. If you leave the expansion area that's accesible in the original campaign till after your finish (and skip the Wild Mage, something you should do on your first run anyway), then your original campaign experience remains more or less same. True you might get a High Level Ability while still in the original campaign, but it's not something that will matter all that much.

Eh, I really miss playing vanilla Icewind Dale. Exploring the Dragon's Eye and Severed Hand without War Chant of Sith is something different... to se ne vrati.

I'm torn. I'm really tempted to listen to your advice to get the original experience; especially with the people above mentioning the difficulty curve getting a bit out of whack with the expansions.

But...

I don't want to risk havig to trudge through the original campaign again, just to try the expansions.

So, my quesiton is: is the game replayable? Without making assumptions for how much the game is liked.

For example: Despite loving Might and Magic 6, a lot more than I liked Diablo, I'd call the latter more replayable: playing a Mage/Fighter gives different experiences as well as getting different quests. Might and Magic 6 on the other hand, despite having different classes, can be experienced "wholly" by having a balanced party and a balanced mix of skills.

How is PoE in that regard?

The only problem with my approach is that the caps are going to be the max if you choices to escalate the difficulty. This is one particularly tricky with the lasts areas of the campaign if you pick a violent approach (probably that is why that area has a lot of tents everywhere even on PoTD settings). But frankly, everything in the game is telling you "go stealth" (you will know what I'm speaking even before arriving to that area :P )

the special battle with Llengrath (which can be avoided) I'm with the idea that is always to max level possible even if you do the expansion first. I could be wrong though.

I have 6 different characters on that game. So, my answer to replay is : "YES, a lot"

The late game is too easy without the expansion. By the time you are ready for The White March (levels 8-9), there are only a few battles in the vanilla game that will still present a challenge. Most of those same battles, like the Adra Dragon, are still going to be difficult by the end of Part 1.

The White March Part 2 is the real endgame content, and is what you should save to do until right before the final boss.

I have bought the dlcs after finishing POE1. How can I import my former party? I've tried to load one of the last save games, but didn't find how to continue with the new content.

The DLCs are part of the main game. You can't start them if you've already finished it. The only solution is to load an earlier save (before the point of no return in the main questline) or start the game once again from the beginning. In this case you'll have access to the White March after you reach a certain point in the main questline.

Thank you! I think I loaded the very last save. I'll try an earlier one. Will there be a new entry in the quest line?

You'll be notified that the White March is available by the stone lady in your keep. You'll also get an entry in the journal related to the DLCs.