I would suggest buying the amplfier and streaming device separately. Do you currently have any AV within your house that you would like it to integrate too?
For example, if you had a Sonos system, would you like to incorporate the 151's into this? Or would you prefer a bluetooth connection?
I found this amp; which is rated at 50w per channel at 4ohms which should suffice. I would strongly reccomend keeping this inside and also investing in some decent weather proof cabling to prevent any shortages.