Welcome to the forum Delta1.
Most Antara owners on here are having positive experiences with the car. Like any car, there are a few exceptions.
The car does not come with a spare. It has a puncture repair kit. You can remove part of the boot underfloor to make room for a spare wheel. This is what I have done.
The DPF stands for Diesel Particulate Filter. This is part of the exhaust system and is there to filter out most of the harmful soot particles that are produced by diesel engines. The problem some people have with it relate to how often the DPF goes through a regeneration cycle which cleans the filter. The way Vauxhall has implemented this, at least in the Antara, seems not to be as good as some other makes of car, but I find it is manageable with a little care. There are plenty of posts on here about this subject which may provide you with more information.
As for your test drive, just make sure you give it a good test and put the car under some stress. Get a feel for how it corners and accelerates. Get on as many different road surfaces as you can and see if you can force the AWD and stability controlto kick in to test stability. You don't say how long a test drive you have, but use whatever you have got to test the things which are important to you in your cars.
I've had mine for just over 3 months now and I can honestly say I'm happy with the car. I get a DPF regen about every 520 miles and my average MPG over the 3000+ miles I've driven since new is 31.