The nvidia package is probably going to be anywhere between 200-550 mb as a fair warning. That has been my experience in the past. I use AMD now though.
I think that size is what manjaro downloads, just a bar on the hardware installer so no MB/GB sizes for the packages downloaded, kink of like windows, it'll be down when it's done attitude with just a progress bar.
I hope the xfce installer/packages is in the ISO also just a version ten dvd iso no package with a specific gui listed which is different since most do that to save bandwidth instead of including them all in one ISO.
Yeah, Still going to need to run updates though. Not on install to at least get working and setting up your env but it's going to need it. My experience is at least a extra gig or two of updates after install unless the ISO was JUST released.
If it's got the open source nvidia then that was 8/6 so it's likely a relatively recently updated ISO I hope since my wifi only gets around 1 mb/s download speed max.