I bought a second hand travel system for £150. It was perfect, but never used after DS was 5 or 6 months old and we could use a basic fold up buggy instead (£40, Mamas and papas - bought from factory shop).
£600 is a ludicrous amount for a pram full stop, let alone a second hand one. Especially as I don't know anyone who used a pram beyond six months - but then my friends are all practical types who need to get in and out of cars, on and off public transport, around town easily, etc.
We also generally live in terraced houses with no hallways in which to store a big pram. So my travel system had to live in the kitchen, creating an obstruction, until happily it could be relegated to the cellar once DS went into the buggy.
If you're just going to push the pram around your neighbourhood, won't be using public transport, have a big house, and aren't bothered about it being easy to bung in the back of the car, then you may use a big pram for longer.
Also, don't confuse what will fit in your car easily with what will be easy and light to get up and down while juggling baby, bags, car keys etc. My travel system was light and fit in the car easily, but was never used as soon as DS could go in the buggy, simply because buggies are so, so much easier and lighter.
Think carefully about whether you can put the Bugaboo down one handed, and open the boot, then put it in, all with just one hand.
It doesn't seem like a big deal now but in a few months, when you're knackered and have a shouting baby who won't be put down, you want the easiest buggy possible - unless you're very wealthy, you don't want to have splurged hundreds on something that never gets used.
Sorry, that was long! In short - £600 is loads for something you may not use much.