I replaced the evaporator in my 96 850 earlier this summer and overall my reaction was that it was not as bad a job as I expected. I'm glad I did it myself -- saved a bundle. It does take a long time because of all the steps but most were very straightforward -- and mostly "clean" too, just unscrewing parts in the interior. I would consider myself an average DIYer in terms of skill and experience although I am fairly slow. It took me 1+ days to finish, mostly because I forgot some things in re-assembly and had to back up.

I'd say you definitely want to follow some good directions to you use the right sequence of steps. I followed those found at: www.volvo-forums.com/t2756-evaperator-replacement-instructions. These are really detailed and accurate. I did find one or two slight deviations that were apparently due to changes by Volvo from year-to-year, but that shouldn't be a problem.

You need to be careful to mark all the connectors etc so you can get the right ends back together -- there are a zillion wires dangling when everything is apart.
And it really helps to have an extra body to help lift the dash in and out -- but it is not hard to do, just too long and awkward to handle by yourself.

As far as special tools, you do need spring lock disconnectors (I bought the cheap set of them at Sears) and the torx drivers. I had to make an extra trip to Sears because one of the torx drivers needs to be especially long -- I forget which. The web instructions have a list of the sizes you need.

Good luck if you do it.