Car potty - if ds used it, goodness knows where he and potty (and contents) would end up. Its bad enough when its a normal potty on a tiled floor (sorry if tmi!)
Grobags - yes, we started using them at about 6 months (but then, April baby, so not really much need in summer) - need 2.5 tog for winter and 1.5 or 1 tog for summer. The real 'Grobags' are very good and wash extremely well - watch out for them in TK Maxx if there's one near you - and also the travel Grobag - you can fix their 5 point car seat harness properly but still get them out in a nice snuggly grobag the other end of the journey (travelling at night is ideal after about 6months as more likely to sleep).
Chair - well, ds would like it, but I got 2 chairs and a table in Ikea for same amount - have contemplated painting them or doing decoupage on table top but not had time. I think that coat of varnish might be as far as it gets, but at least it would wipe off more easily then.
Bags - don't get Avent rucksack version - looks good in principal but very uncomfortable. However, great to have a bag with a change mat incorporated and in place that you can get it out one handed in a tight place with no where to put baby and/or bag down.
I found a feeding pillow really helpful (Blooming Marvellous) - particularly as had emergency section, but also as ds so heavy and wouldn't have wanted to support all that weight all the time.