I'd recommend buying over renting if you aren't going to use it much. I hired a capsule from QLD ambulance and loved it. Was much easier to get bubs in and out of the car when you just have to unhook the insert, esp when bubs was asleep.
Getting a seat for JIC moments is a really good idea, plus it can be passed around or used for children later on. Buy one that can last a very long time, ie birth to 18kgs. That way you aren't having to buy another down the track.
Good luck with it all and congrats on your green thinking. Using PT all the time is fantastic, wish I could get away with that here but live a bit far away from everywhere.