The problem you have is you need someone who knows both systems and has migrated previously in the same direction as you. PostgreSQL doesn't have a massive following I'm afraid.
I imagine there are commercial products out there that would help, other than that some concerted googling to search forums such as this.
I assume you know getting from SQL 2005 to SQL 2008 is trivial though yes?
I migrated some databases from SQL2000 to PostgreSQL about 4 years ago and it was all a manual process. I found a few freeware tools that purported to output your SQL Server db schema in PG format but it didn't cut the mustard for me, it got things wrong all over the place.
I ended up using SQL server management studio to create a script for the creation of all tables and views and manually adjusted them all to the correct syntax so I could run the script on PG. It wasn't too much work, took me half a day to fix up a script for about 500 tables and views.
Functions is a whole other kettle of fish... I had to rewrite all of them from scratch which was a few days in itself.