I haven't traded dialup in a long time, but IB's TWS required very little bandwidth (less than any other realtime platform) when I did use it last. I have no indication that this has changed. It's primarily because they don't send every tick, which is a great advantage when the bots start fighting each other for the inside market, something that swamps my machine and 3 Mbps cable connection to another broker if it happens in more than a couple stocks at once, while IB's app running on another machine on my slow DSL connection quietly displays a snapshot a couple times a second (and I see no significant increase in bandwidth utilization on DUMeter - neat tool).
You need a margin account to avoid the settlement delay issue. Most brokers (correctly) follow the "no trading with unsettled funds" rule for cash accounts, and those funds settle 3 biz days after the trade date. Just because you have a margin account does not mean that you have to use margin. There is no downside I can think of, other than personal discipline.
As far as support and reliability, I can only suggest that you try for yourself. Remember that people hardly ever say anything when things work well - most of what you read are the complaints. Some people have few problems at all. Particularly with low-frequency trading, you are unlikely to run into problems. Worst case, you switch somewhere else. For the most part, application, funding, and de-funding if necessary are all electronic and fast.
