There aren't really any disadvantages, no.
I'd advise you don't get a broadband package with a limit though, because the most annoying thing that could occur is for you to download 2gig of data (even from surfing web) and it'll cut out saying you can't use it anymore.
There are so many companies its hard to really say whos best and whos not, though I've heard Wanadoo are alright, and they do do a Broadband package, though I'm not sure if it has a limit or not.
As for the question about adjustments, the answer is no.
The company should give you a new card for the faster speeds which should just plug into a PCI port, and everything then should go fine.
Since you are running on a rather slow connection at the moment, going with 1mb would be alright for you, and since the speeds are increasing now, it shouldn't be that expensive either.
You won't regret it though. Theres nothing better than downloading something, and it only taking a few minutes to download a large file.
All you have to worry about then is your hard drive space limit
I hope this helps.