You can manually install the latest flash apk, and some flash content will work, but some flash content will also crash out the web browser.
I know from experience*, the flash based livestream.com video streaming applet works fine,
But farmville crashes on load.
So it can be hit or miss depending on what you are trying to do.
Cheapest current dual core android phones are:
Galaxy Ace 2
To search for more, trawl the wiki product pages, looking for dual core CPUs:
http://en.wikipedia.org/wiki/List_of_HTC_phones
(There are more options, I can't think of them right now and hopefully i have given you enough suggestions to get started)
*mother in laws phone. I tried rolling back to ICS, 4.0.4, the last officially supported android With flash os. I also tried rolling forward to JB 4.2.2 Didnt make any difference and flash was still unstable, hence why threy dropped it. Its a POS
Edit: If its for watching video streams, would a tablet be better suited?