Currently using a 3300v with ADSL from two different ISPs. Both going through a vigor 100 first. No problem at all and works well as a load-balancer.
We have been using a bonded solution for the past 18 months from EXA Networks. Currently we have 2x2mb lines but am looking at upgrading this to 4x2mb in september as we need more upload speed.
BT have stopped supporting bonding using some older protocols, Managed Communications can offer a fully support and BT apporved bonded adsl solution.
I think that the simple answer to the original question is yes, the Draytek will load balance across multiple ADSL lines and that if you choose different suppliers that would be a better option. You can use the Draytek 100/110 as the PPPoE ADSL modem and the 3300 will treat each connection as just another connection. I have recently noticed that Griffin Internet do what they describe as a Engineering level ADSL connection that gives 2MB symmetric connections that might be useful if you have people dialing in or you send out a lot of data. Bonded connections are another option but if you are using multiple ADSL on one router you are achieveing the same thing and do not need to go the bonded route if you feel aprehensive about reliability although I cannot say I have found them to be necessarily less reliable.
If you need to NAT various services you can NAT using either IP aliases or WAN ports.
If you need a UTM device the 5510 is a good option but only has two WAN ports.
The draytek works well using x21, ADSL, SDSL and 100/1000 MB fibre and is a good no-nonsense device. The only pain is that the username for admin is set to dyatek and at time of writing the firmware does not allow a change.
The legacy way of bonding requires the telco to support MLPPP (multilink PPP) which BT stopped supporting. The only other option to truly bond (not load-balance) is to use "broadband bonding". Broadband bonding is a technology that does not require the telco equipment.