You won't find the GS as solid or well built because it isn't, although 2000+ models seem better. Rattles, wind noise and fidgetty suspensions are my complaints although the first two are mainly apparent because the car is so quite that you notice everything. Suspension on the 430 seems much better, maybe the 300 is better now although I haven't driven anything newer that 2001 model.
You do get more space although rear headroom is not good for anybody over 6ft, hopefully your kids are not that big!
If you have an auto IS200 then you shouldn't notice an increase in fuel costs.