You should be able to setup a PPTP VPN server on your Windows Server at your office.
I highly doubt that your ADSL modem will be able to establish a PPTP VPN connection over its PPPoE connection to your ISP.
Most routers would try to establish the PPTP VPN over Ethernet, which won't work for you.
Letting your whole home connect to your office via a VPN isn't recommended, unless your office network is well protected from the inside/VPN clients.
If you really need to give your whole home VPN access to your office, then you can always buy a MikroTik RB750 (~R400 incl VAT, ex shipping) and establish the VPN from it to your office.
With the MikroTik RB750 you can give everyone in your home access via the VPN, without routing all your other Internet traffic through the VPN, but for this you'll need some knowledge about IP routing - but we'll be able to assist you there if you need help.
For a start, I would suggest that you setup the PPTP VPN server at your office and then just try to establish a connection from your own PC, before going out to buy a router to connect to the VPN server.
Lastly, if your office does not have a static IP address for its Internet connection, then you'll need to setup Dynamic DNS. The MikroTik router's don't really work well with Dynamic IP addresses, so you'll have to script something to update it every hour or so.