So this thread will answer the antenna for you:
Radio antennae - Page 3 - RIBnet Forums
Radio gets slightly more complex. There is a recent thread here:
Which fixed VHF, ICOM or SH?
However you probably need to figure out what you want to do with the GPS before making a decision on the VHF. If you want a Long Lat then a VHF with an inbuilt GPS does the job. Standard Horizon do one. Not sure if it would list speed but worth a look.
But a Long Lat is not really helpful at 20kts on a Rib! If you mean you want a plot on a chart then do you want colour? and what river because you'd want to check the chart covers it...
If you aren't strictly navigating with it would u be better with a smartphone running navionics in a waterproof case. Which brings you back to the VHF having inbuilt GPS so that the DSC has a possition...