First and foremost, you should really make sure you are familiarized with the inner and outer workings of a home. I realize you might just be there to trash the place out and clean it up, but you most likely are going to need to provide estimates for damages, even if they are just eyeball estimates. Here is an illustrated reference book that I used when I run into something that I'm not quite sure about:
I also use the illustrations in this book to train my employees. It has some good examples of different wet heating systems.
Now that you've identified the problems in the house, you'll need to write the bid. If you don't want the prices on your bid slashed by your client, be sure to include some detail on them. Here is a good site to get verbage on different preservation related jobs:
Just remember to write out all of the steps necessary to complete the job your bidding. For example, if you are bidding to replace the water heater; make sure you are adding to reconnect the venting, gas line, and the supply lines to justify your price.
I could write an entire book on bidding, but I thought I'd share a couple of resources here.