If the exhaust manifold has not been done it'll prolly need it, so better to go ahead and do it. If all studs etc are intact, with a bit of very careful removal, (cannot emphasise enough HOW careful) you can take it off get it refaced and grind all the webbing away, then replace it.
Beats the nightmares a lot of us have had, me included, of snapped off studs etc!
If that quote includes removal of the fool tank, it looks a bit too good to be true, but then maybe it isn't as extensive as others have been. And make sure he cuts the rot out rather than plating over it, and he does it with thicker plate than original.
My bloke charged me £200 plus Vat, (I think) and he struggled with removing and replacing the tank. which he was decent enough to not charge me extra for.