You don't exactly want to download all Modland. The archive is huge, and there's, seriously, way more material than you will ever hear, and if you do, 80% of it is crap.
This comes said from someone that took the ProTracker folder of Modland and heard it in its entirety. More than 6000 mods.
It's much better to just download what you like, when you need it. As a means to discover cool new music, you will discover indeed that there's too much crap
(Modland is organized by artist and by format. That's pretty much all you need. There's also music for MORE than just Amiga, like consoles and shit. I repeat, you don't want to download it all. But use an FTP client, not a web interface)