Of course this sort of report is very upsetting especially if you are not a person who has knowledge and experience of cars? Sorry, I don't know how "technical" you are.
Let's consider what has been reported here. By the way, what year is your car and what mileage on the speedo?
Rear axle corrosion is common on many vehicles fitted with this type of rear axle - our Panda (2010 1.2 Dymanic Eco) will soon be needing a new one for this very reason. It's the degree of corrosion that's important. Our's has been rusty for years but it never failed it's MOT on it. The MOT tester is obliged to mention it though. So you need your garage to give you their opinion as to how serious this corrosion actually is - It could go on for years as it is.
Exhaust systems start corroding from the day they are fitted and especially if it's a fairly cheap aftermarket system. So it depends again on the degree of corrosion. Rear silencer boxes can look absolutely awful because many are double skinned and the outer skin can start to peel away when rusted. This outer skin can happily be cut away and the silencer will continue to work satisfactorily for at least another year or so. It's disappointing the clamp is missing though.
Tyres on 2mm? Unacceptable on a car which has just been bought. The legal limit is 1.6mm but I wouldn't be running with less than 2mm on one of mine as wet weather performance will be much reduced at that. The selling garage wasn't breaking any laws - as long as there was no actual damage to these tyres, but it's very disappointing they sold it to you like that.
Sight corrosion to front discs and pads half worn, no problem with that I'd say. I think your garage has just mentioned this for your general info as it seems hardly worth a mention. Might be worth checking if the front brakes are running freely though as they can get a bit "sticky" if not properly maintained. The "clunk" is quite another matter and needs to be investigated urgently. I'd not be driving it until either this is fixed or I know that it's not a safety issue.
So, I think it's very disappointing that the selling garage hasn't properly prepared this car for sale. Maybe they haven't really given it much of a check over at all? However it may be that this is just a wee car needing a bit of care and attention to make it a good proposition? A lot is going to depend on whether that back axle is severely corroded or just has slightly more than surface rust on it. That's the potentially expensive thing to repair if you can't do the work yourself.
Is your garage a "Fiat orientated" workshop. If not a Fiat specialist it might be an idea to try to find one and get them to give an opinion as they will be used to assessing the condition of the rear axle. Or maybe there's a forum member near you (roughly where in Yorkshire are you) who could take a look at it for you and give you an opinion?
Reporting a suspect MOT test? I've never done it but it can have very serious implications for the garage (rightly so if they deserve it) My workshop lost it's ticket briefly - due to a "silly" misunderstanding - and it took us ages to get reinstated and made me very unpopular with my management! However I believe dodgy MOTs do need to be questioned to uphold the standards. This may help?
https://www.gov.uk/guidance/report-an-mot-tester