Consider this:
Could you run the same computers on the same LAN at 10Mbps? If so, then that tells you something about the speed you need. Most internet connections run at or below this speed for download. However:
Don't rely on ADSL speeds as the upload speed will be too limiting. Interoffice VPN speeds will be limited by the ADSL upload limit. That is generally too slow to do anything but very small file transfers in a "normal office network context". i.e. people will be so disappointed that they won't use it.
Consider an MPLS or similar (maybe fiber) link between offices - the names vary. Get it from your ISP if at all possible - or utility company. T1 isn't well suited for this much more than 56K frame relay!! Just too slow. Whether you call this a "leased line" or not it's the likely direction you should be thinking about.
With a dedicated connection then you may be able to avoid setting up a site-to-site VPN - as it may be treated as a private VLAN. But, if you decide to run a VPN it will entail some overhead and you will lose a fraction of the bandwidth. I don't think the VPN encryption bandwidth loss is worth worrying about. Either you have adequate bandwidth or you don't. 20% won't matter.
It's possible that you will need to have each site on a separate subnet. Absolutely if you use a VPN connected between each site. If the inter-site link(s) interface looks like a layer 2 switch then maybe you can keep everything on the same subnet. This is a key point in the architecture.
