Welcome back!
My advice is find out what is cheap or available on craigslist in your area. Let's face it if you could get an Amiga 1200/3000/4000 for a good price locally that is probably your best bet. However that may not be a possibility.
Since you live in North America you will notice that the Amiga's above are actually not all that common. We get allot more big box Amiga's like the 2000 and quite a bit Amiga 500s
The good news is that the almost any Amiga is expandable now thanks to some really awesome expansion cards produced by hobbyists.
For the Amiga 500 I agree that the ACA 500+ is your best bet. However there are some really good and cheaper products at kipper2k.com which will allow you to expand the memory and install a Flash hard drive.
For the Amiga 2000/3000/4000 there is the SCSII2SD device that allows you to hook up an SD card to your SCSII port in the big box Amigas.
To copy your files to and from Amigas there is an awesome free product called Amiga Explorer
https://www.amigaforever.com/ae/
