OK for one thing, the car in the pictures does look like it has alloys....just not very nice ones.
Second, 18k on a 3 year old car is very little but I don't put up much more than that and many people probably put up less.
Now, if you meet a seller ANYWHERE than his own address, then forget it, it's a bad omen.
He sounds like a decent enough bloke offering you the diff alloys and so on, so there's the other side of the coin.
However, as others have said:
Meet him at his house or place of business ONLY!!!
Ensure that the name on the ownership docs match his name.
Ring the previous owner to see what their reason for selling was and roughly what mileage was on the car when they sold it.
Check the level of wear on the seat bolster, steering wheel, gearknob and pedals (18k should not make any impressions on anything but maybe the bolsters and only then if the owner is overweight.
If he's a dealer get a guarantee (even 3 months) or walk away. Make sure it's a comprehensive one too!
If he's a wheeler-dealer, WALK AWAY!!! If anything goes wrong (and it may) he's not gonna want to hear about it.
DO NOT bring money to the next meeting....bring an AA engineer (couple of hundred well spent, considering the price of the car) and if all checks out with him, then you're OK if the Cartell check comes up ok too.
But above all, keep looking at
www.autotrader.co.uk and see what kind of car you can get for your money under 50k miles...as that's only running-in miles for a modern diesel anyway.
I wouldn't even worry about the taxi/reps car comments. What? Do only Taxi drivers and reps own Passats/Octavias/Avensis now ?