Hi, JJA!
To answer your question, first I assume that you will be doing it on G6.x right, and not G5.1. I will try to write about disadvantages of trying to use G5.1 outbound dialer. If you MUST use G5.1 then you will need something called SDialer which comes with Genesys. In 5.1, SDialer is very buggy and its functionality varies from one version to another; just take a look at the release notes to see what I mean!
In order to migrate from Avaya to Nortel, unless you have some really fancy hardware, you will need to use something called Call Progress Detection server.
You see unlike Avaya, most of the standard Nortel's implementations do not have Call Classifierlike module installed, so you will need to use a thirdparty products to do the job for you.
Call Progress Detection (CPD) server is part of Genesys Outbound Solution. It is installed on the same machine as the dialogic (or other voice cards) that will actually be placing the call and trying to identify whether or not the call was answered by answering machine, human, whether or not it is ringing, busy or out of service.
From Genesys perspective, it is really easy to install and set running, and after a while you no longer even aware that it is there. The real problem lies with the fact that you will need:
1. voice cards (Dialogic?) which will be connect to your PBX via T1 (preferably)
2. if you will not use T1 to connect your cards to the PBX then you will not be able to use mute transfer and instead you would need to do the old fashioned twostep transfer, which will add at least 3 seconds to your overall time oconnect, and on top of that, make your overall system prone to transfer errors, because dialogic cards are venerable for their sudden failure to transfer a call.
3. Also, you need to keep in mind that number of calls in an ACD queue is never correct, unless you use URS and virtual queues. It has to do with inherent Nortel limitation which prevents Nortel from issuing an EventQueued via CTI link until call actually leaves ACD queue and arrives at its destination.