Hello & welcome
I don't claim any of what follows as gospel - but here's my take on the series 1:
I think I'm correct in saying the series 1 was available thus:
LS - 2.4 petrol - 2WD - manual - 5 seats
LT - 2.0 VCDi - on-demand AWD - manual or auto - 5 or 7 seats
LTX - 2.0 VCDi - on-demand AWD - manual or auto - 7 seats
I have an '08 plate LT: diesel, manual, 5 seater. Not the most refined engine, but torquey enough for my needs and economy is ok for a big SUV. Tows our caravan (1200kg) fine. Rides & handles well enough; plenty of load space, cubby-holes etc for storage; interior quality is fair rather than outstanding, but then it's not a Range Rover. As Aussi Ed says, it's a lot of car for the money.
The diesels have a DPF - so be aware of the potential issues. If you went for a 2.4 petrol, I'd expect it to be both thirstier and slower.
Personally I'd walk away from one that didn't have a full service history (and ideally a wad of receipts to back it up) - oil changes at the correct interval and the correct oil spec are essential.
If you have a browse through the forum's Captiva section, you'll quickly get a feel for the most common issues. Aside from that, just check all the things you'd check on any used car...
Hope that's some use.