Something that's a determining factor, is how much data you need to transfer, and how often. The ISP you choose doesn't really make a difference, since they all cost about the same, with R10 - R20 difference here & there. I would suggest you take something that's unshaped, and setup a VPN tunnel between the two connections, cause then you can use normal windows browsing, otherwise you might use email, if the files aren't too big.
heh, I had a problem @ one client yesterday, she phoned me and told me her email is slow, and I should fix it. Working on the same mail server, my email was quick (I use 384K ADSL, and they use 1024K ADSL!), so I went over to see what the problem was, and someone send them a file that's 44MB BIG!!!! So, if you do want to use email, keep it below say 10MB per email. You could probably also use FTP / SFTP for this purpose.
With 99% of the ISP's you go with, you'll get a dynamic IP address, so to make it a bit easier, use
http://www.dyndns.org to setup a a fix hostname(s) (like sa-pc.dyndns.org & uk-pc.dyndns.org) for the dynamic IP's, which will make it easier to get to each PC.