I'm not a local but have been there annually for the last several years and will be visting again in a few months.

A few replies:

1) be careful with money - dont change it in the customs area and go to the banks to get a better exchange rates
**I think there's the Banco De La Nacion which has a window to the customs area (sometimes open) and just outside of it that is good for exchanging money. Make sure you hang onto small bills - that's a lifesaver for making transactions go smoothly.

1) Is it easy to get along in these two places with English only?
**I speak very little broken spanish from high school over 20 years ago. I get along fine. Most who cater to the tourist seem happy to offer up some English. You might want to download a simple offline translator to your phone which can be handy

2) How careful do I need to be with my valuables (I have been told about pick pockets but I am assuming hotels are generally safe if I left things behind)?
**Don't bring anything out with you that you don't need. I always leave my things in the safe in the hotel. I usually only bring one credit card and not all of my cash.

3) What should I use to travel around the city? Cabs? Trains? buses?
**Cabs are fine and I've found the vast majority of drivers to be very friendly and helpful. I try not look at the road too much because the driving in BA terrifies me. Subways are easly navigable. I've not been adventurous enough to take a bus

4) Ride from the airport. Anything to be aware of? Should I have this booked through the hotel?
**You can use http://www.tiendaleon.com.ar when you arrive at the airport. They are very efficient and reputable. Many on this forum have also used http://www.dantesairporttransfers.com/ and I used his service several times on past trips.