Car seat and Zipcar

If you can pick up the car and bring it home then it's more feasible to use a bulkier or heavier car seat possibly using isofix.

How does it work with picking up the car - do you need to travel to the destination with toddler in tow or could you pick it up, drive it home and install seat?

I would max out the infant carrier as they are so much more portable. Then get something like Joie stages, or Joie Steadi is a bit lighter and was on offer recently, or britax first class/britax eclipse is a good choice if you think you're likely to forward face (britax eclipse is also plane approved).

Carseat we have for ERF is seatbelt only but it's a pig to put in and out and make sure it's secure

Yes, of course it would be (I haven't done it with Zipcar, but I have used a car seat in a hire car, which is the same thing). All modern cars will have some sort of isofix/i-size attachment points, but it's hard to know what sort of car you'll get (I don't have an i-size car seat, but supposedly they are 'backwards compatible' with isofix). It might be more straightforward to simply get a car seat that can be put in with a seat belt, or alternatively, I would call or email Zipcar and ask what is standard in most of their fleet.