What you really need isn't going to be as easy as setting up a NAS or EXT hard drive.
You NEED to have a dedicated server box. It doesn't have to be all that great of one either. Dual core CPU, 2GB ram, 500GB HDD, integrated graphics and you're on your way. You'll need a copy of Windows Home Server of some flavor. You could use one of your desktops as the server if money is tight. But I prefer not to perform daily tasks on a server/working box. Ideally, you want that server box sitting out of the way and only touching it when installing programs and doing updates.
Once the server box is set up, you install Quickbooks Pro on the server as a server install. I don't know anything about QBPro, but I would imagine it's capable of running on a server and being accessed by two or more other terminal computers, ie...the desktops you are currently using.
Since this is a business computer setup, you NEED to have at least these two other items.
1. A good, name brand UPS device.
a. CyberPower CP850AVRLCD
This would be good for a desktop with monitor, back up hard drive and other small devices. You at least want your Server box plugged into one of these.
b. CyberPower Intelligent LCD UPS CP1500AVRLCD
Or you could go with this one and run not only your server box, but your other two computers as well. As long as one of those other computers isn't a gaming powerhouse!
c. Whatever brand or model you decide to buy, make sure it has AVR (Automatic Voltage Regulation). AVR will help during brown outs, blackouts or over-voltage due to power station fluctuations or lightening strikes to sub-stations, etc. Also, note that a True or Pure Sine Wave UPS is more expensive and a better option on a UPS. However, most decent, good quality UPS's will use a modified square wave output. This is OK and is much less expensive. I use this type in my own home and I've saved my computers at least once for sure. A carpet cleaning van struck a power pole which had one of the streets main transformers mounted to it. As a result, the whole damned neighborhood went off/on/off/surged back on then off for good. All of my electronics are UPS protected and I didn't lose anything. However, the guy across the street had to file a home owners insurance claim because his brand new Mac desktop and brand new, two day old Mac laptop got badly damaged. Along with his expensive LaserJet Color printer. NEVER plug a printer into a UPS with your computer on it. Use a high quality power strip for that. Printers, especially lasers, have a very high peak start-up voltage that can, at best, shut off your UPS. At worst, it will fry your new UPS.
2. You will need a good backup solution that is external of your server box. You sure wouldn't want to lose your data!! That would really suck come time for taxes. Seagate's Blackarmor products are outstanding. The backup and scheduling software they come with is, IMHO, the best you can get for running a business.
3. A NAS or backup device alone will not allow you to both access the same program at the same time. So you will still be stuck waiting for your wife to get off the computer so you can input your data into QBPro.
4. You will either need to make a thread here asking for all kinds of advice on how to set up a home server, buy a book telling you how to do it, research on the Internet or, my suggested option, hire an IT guy to get you setup and running. After that, you won't really need an IT guy any more as long as you know how to do basic OS and program updates.
I hope it all works out for you soon. I know there are cheap boxes from HP, Compaq and the like that are already setup to be a home server. I'm just not sure how suitable they would be to a business environment. Dell has a pretty good system as well for the home business and their technical support, once you get through to them, are pretty decent in telling you how to get some of the things setup. Still, you will need to figure out how to properly install QBPro and I know Quickbooks technical assistance can cost you some $$$ to help you out.
Good luck! I'm sure others will swing by with other advice for you.