My friends and I are looking to get back into Descent and we just have 1e. We had planned on getting the campaign exp but then found out about the 2e would these be cross compatible? Because we would still like to attempt a campaign of sorts also how about the conversion kit can you just get that or is it just to play with your old fav's in the new world?

2E is strictly a stand alone game and is not compatible with 1E.  The conversion kit however allows you to port over your first edition plastic to gain access to the many monster and hero options to expand the possibilities within 2E.  Personally I, purchased the conversion kit, even though I don't own the plastic figures from 1E, to maximize the available options and simply am proxying pieces until I figure out how I want to permanently rectify the lack of plastic figures for specific heroes and monsters.

some people I know are working on D1 to D2 conversion.

I'm not sure to understand.

Figures are already converted.

Except tiles, what else could be converted ? Class, overlord's cards, and else are totaly different. So, what's the idea ?

 Thanks for the answer, we are likely to try and get our hand on a road to legends if we can find one that won't make us poor.