Hi, I'm about to implement 1 Draytek 2860N and 2 Draytek AP-800's in a new office.

Its the first time I'll need more than one access point and I'm looking for advice on the best method of setup.

Same SSID on all 3 devices? what channels? I've read a little about "hand-off" - any advice here?

what would you do? I don't need any VLANS etc, just one accessible wireless network around the building.

many thanks

If you are going to use the Managed Wifi then its advise you done use the Built-In Wifi from the 2860. You can include itself in the managed profile.
Once you set the profile up you can leave the channel on auto.
I am not sure if the AP800 have the Roaming feature like the 900s do.

The AP 800 does not have roaming but set same SSID, user name & pass, channels, also at the AP you can try turn off the DHCP at LAN >> General Setup > DHCP Server Configuration
At Operation Mode Configuration can try with universal repeater
http://www.draytek.com/index.php?option=com_k2&view=item&id=2777&Itemid=293&lang=en

You can also try DHCP enable with range 192.168.110 - 192.168.1.200 at the first AP & disable DHCP at the second AP

There are quite a few articles in the knowledgebase on managed wireless now.