Pickle - in the kindest possible way, I do think it would be daft for you to make such a big purchase at this stage.
A few of us on this thread are showing that it's not always straightforward. I would advise going into this expecting it to take multiple attempts to get pregnant. Time creeps on with every attempt.
Just from a totally practical point of view, every year the safety of car seats improves so it makes sense to buy the car seat at the last minute.
On more of an emotional level, I can't help but think that the moment you got pregnant you would start imagining putting that baby in that pushchair. If you miscarried, you might regret not being able to do everything afresh for your next pregnancy.
Another point, I would never buy a car seat over the internet unless it was to order it via a reputable retailer like John Lewis. The safety of the car seat trumps any £50 saving on a purchase through eBay.
Sorry - I am being very blunt
but in short, yes I do think it would be slightly insane!
I'll be interested to see what the others say because I know that I am particularly cautious and it might be that I'm being a kill joy!