I would say, if you've never even sat in a GTV before (hence your question as to will you fit) and obviously you've never driven one before, then how do you know if that one is going to be a good or bad example even after driving it ?. Without trying at least 2 or 3 (better still 5 or 6) others your measuring everything on a sample of one
At times there's chalk and cheese difference between 2 cars of the same model, and you stand a chance of being dissapointed with the 'model' when its only that one car which is wrong. Conversely if you think its really good, remember that there could be others for sale that are even better.
At 6' 1" with my shape I find I have to slightly 'duck' to see fully out of the windscreen, i.e when close to traffic lights etc.
I personally would be suspicious of new tyres. For the most part the rears are easily seen for the amount of tread they have, and I would never put a car in for testing with such an obvious fault. Similarly for the MoT station to turn it down there has to be significant wear across a % of the tread, possibly on the insides of the tread which are harder to see, and indicative of suspension wear/settling.
At £3K its certainly not cheap for 10 year old car, noting that what people ask for and what they end up taking can be significantly different, and if the rear end is on the way out, the figure £500 springs to mind for a rebuild.
Please note, this actually isn't to put you off, it's just to stop your heart running away without your head attached
PS It's not a 1999, the service book has a fiat stamp in it dated 15 Sept 1998.
PS How long has this guy owned it ? does he know its real history as its been around the country a bit, Manchester, Chemsford etc