Ok, you don't need more than a couple of Mbps for streaming from a Solo so you can go for the cheapest.
Even the HD Slingboxes don't use more than 8mbps so your 20/3 package should be fine.
However, watch out for any data usage limits. Some ISPs will slow the connection down when you hit a certain usage in a month but these should be clearly advertised.
Let's say you were streaming at 2mbits/s, that's 120mbits/min or 7200mbits/hour or 900MB, so nearly 1GB each hour. So if you had a cap of 50GB per month then your Dad would be able to watch 50 hours per month before hitting the limit (that's if he didn't use the Internet connection for anything else too!)