704 being the software version? what is the most recent one and what are the differences?
805 is the current version, but 704 will do everything you need on the A3. There are some additions to the later versions but mostly for other models such as the new A4 A5 and the other all new models. With the genuine lead you also get support from RossTech and can email them log files and questions on errors that appear, however you'll probably find that somebody on here will be able to offer assistance should you run into problems that you can't solve.
Even if you only use it the once and it costs the same as an hour's labour then you are not out any money. You can then continue to use it or else sell it on for pretty much what you paid for it if you don't think you'll use it any more.
I originally bought one from TerryMcG in order to install my RNS-e and didn't think I'd get any further use out of it but I use it all the time for various things - most of my recent mods have required some VAGCOM coding. I suppose you could argue that the lead has cost me 20 times what I paid for it because of all the mods I wouldn't have done had I not had the lead in the first place. Steering wheel, coming home lights, sds, video in motion, emergency lights, clearing various errors that were thrown when I was performing other mods.
I've also been able to use it to install nav and bt in my dad's GTI and cruise control and some other bits in my brother's one. I've also done some bits and pieces for friends with A3s, TTs and Golfs.
I've got enough use out of it that I decided it was time to upgrade to the genuine lead and make a contribution to RossTech's R&D costs and have sold on the old lead.
I'd recommend going for the lead from Terry if he has any left and give a go at doing the job yourself.