ISPs don't really care if you run servers, at least in my experience. Probably the worst that will happen is they will ask you to stop, but even that is unlikely unless you're doing naughty things.

File syncing can be a pain, but a slow running rsync --bwlimit can transfer a lot of data over a long period of time, without saturating your connection.

I run a server off my home connection using an old laptop. Works great.