1. setup sounds good, the test will come when you're connected
2. i never had to mod my rc.inet1 in slackware to get dhcpcd to work (except to add an -N option for ntp so it wouldn't overwrite a config file).
3. i've heard that motorolla makes good cable modems (surfboard?). i'd probably research and get one of those, or maybe toshiba. charter might even provide one for you free of charge, or for a small charge.
4. maybe check internet/broadband sites for provider reviews
5. my broadband is on a separate cable from the tv cable (roadrunner). i didn't know they could be on the same cable, but i don't really know a lot about that end of it. i was pretty sure they couldn't, though.
6. disconnecting is of course 100% foolproof.
but that won't help you when you reconnect and aren't protected. for less drastic measures than physically disconnecting all the time, a cheap broadband hardware router can offer good protection. i have one friend who says he was hacked through the router (i.e., the router was hacked), but i've never heard of that happening anywhere else. and iptables is always available, and is good to know anyway.
hth