You need a roof person good at tiled roofs to give you an estimate. Could be a few hundred, or over 10k for full replacement.
I am also troubled by your plan to install a Chinese vent (up draft). I am not sure it is compatible with the current kitchen configuration. You will have put a different vent pipe, that means to break some walls and ceilings. Also it is better to remove the current vent and cooktop, because otherwise you have a useless downdraft vent, does not look good. If you don't take it off, I bet you will hate to look at it when you live there. Then probably cabinets has to be modified too; now you are almost talking about a full remodeling of the kitchen... A thousand dollar vent now becomes a 30k kitchen job. It is almost impossible to modify part of a kitchen without create a mismatched place. If you want to do it without changing the whole kitchen, better bring someone who will do the up draft to take some measurement to make sure you can work out something.
Other things are small (total of a thousand or so) that can be negotiated with the seller.