You can "freshen" an otherwise OK carg all by yourself for less than $100 including a can of Chem-dip (use gloves!!!!!) and replace all the parts subject to wear for under $200.
However, if the throttle shaft bushes are shot, you are looking at a job that can be a little complex ranging from oversized shafts and reaming of the bushes (if they aren't too horrible) to milling out and replacing bushes then reaming to size after.
If anything else is destroyed, new parts are fitted (indeed, new bodies can be had from Burlen Fuel Systems and that is where I get my parts).

The carbs don't have a lot to go wrong and really go together one way. With a couple of logical games you can do one without any instructions whatsoever if you are clever. The book helps a lot, however!

Personally, I'd start by a thorough sluicing with carb cleaner (tune-up in a can!) and adjusting per the Volvo manual AFTER a complete tune up with new parts. That way you can be more certain (nothing is 100% in this world) that the carbs are really the issue. Don't forget the oil in the dashpots... if you have never had them serviced you may want to pull the covers and pistons and dump out/sluice the dashpots first.

Mike!