Finallygotabeltfed is the resident NES 1919a4/a6 expert. He's the guy who helped us put together all of our 1919a4 kits at the group build a while back.
That Atlantic Arms unit you linked to seems to be a little bit high priced - the last time I checked on
www.1919a4.com - there were 1919a4's for sale there for around $2k. The Atlantic Arms one comes with the tripod though - so that's like $250-$300 right there.
The least expensive route to go is to get a kit - and either build it yourself - or send it out to get built. There's a guy right now on 1919a4.com who has around 30 kits for sale - and if I remember correctly the price was in the $900-$1000 range. Supposedly these are high quality kits that have been sitting around for a few years. Building the kit will run $500-$600 from a good builder if you want to send it out. You'll also have to purchase the side plate - which is the serial numbered part - this will run about $185 -$200 for a ready to go part:
http://www.halomfg.com/Results.cfm?category=8
There's a number of places that sell 1919a4 side plates, the parts kits are drying up but can still be found, links for them are pretty commonly available, repro tripods are also pretty commonly available - etc.
If you're curious about the Atlantic Firearms unit - I would call them and ask for details on who built it, whose sideplate it was built on - etc.
Two good sources for 1919a4 information:
http://www.1919a4.com
http://browningmgs.com/
http://www.beltfedshooters.com
If you're short on cash I would get a kit - or two - sit on that until you can afford the sideplate(s) - and then find a builder to put it together for you. I've got two kits right now that I need to get built - and I'm probably going to send them out. I just don't have the time to do it myself lately.