I've been working on Micros systems for about 14 year and the 3700 almost exclusively for the last 10 so I can help with that side of things, but I'm not familiar enough with the other POS systems to offer anything.
Micros 3700 v4.0 - 4.9 will run on XP or 2003 Server.
3700 v5.0+ will run on Win7 Pro 64 bit or 2008 Server.
9700 will run on 2003 Server. I haven't loaded it since they switched to SQL Server so I'm not sure if a redistributable SQL load installs with it or if you have to provide it yourself.
Running in demo mode won't require a key or dongle. You'll only need one if you want to get a license, which is pretty pricey if you're not sure this is going to be a permanent thing. As far as I know there's no northwind-like database. There was an option back in the day to install a demo db, but the 3700 has branched out in so many directions that it probably wasn't possible anymore. Again, the dealership may be able to help you out, especially if you offer a decent price on custom work.
I have a bunch of incremental extractions running. You'll need to make a custom table to hold an identifier for each extraction you write and the last transaction sequence that was exported. Your query will store the last posted and last system transactions in a couple of variables, export anything in that range and then update the last posted transaction in your table. The best idea is to work your way through the code in the posting sprocs. Totals are posted incrementally throughout the day, so these procedures do something similar, just updating the table that hold daily totals instead of dumping data.
At the risk of sounding like Morpheus, the quicksand is as deep as you want to make it. The database is crazy, with over 1500 tables total and 255 that hold details use to post totals. Granted, you'll probably never have a system that uses all the tables, but there's definitely a learning curve involved.