You definitely want to look at SATA 3 SSDs. SATA 2 is old and outdated by now, and most modern SSDs, even budget ones, can max out SATA 2 interface without problem. Looking at your budget, it's pretty tight for a 240GB SSD.
The only SSD that I can think of that is that cheap is probably the
Trion 150 or
PNY CS1311, which aren't unreliable or outright horrendous SSDs. The Trion 150 just has abysmal Random read/write performance compared to other SSDs. The performance gain that you see going from HDD to SSD is mostly because of the better random performance of an SSD when compared to an HDD. The Trion 150 is just a bit above a good HDD (It's better, but not by as much as expensive SSDs). The CS1311 is basically the same in performance.
Overall, you get what you pay for. An
850 EVO is just so much better than both of these budget options, and the cost reflects that. If you really want to feel proud of your purchase, I'd recommend waiting till you can afford it. Also SSDs to consider (which I haven't looked at reviews for yet) are Intel 540s and Samsung 750 EVO, which are new budget offerings from Intel and Samsung, the two big players in SSDs.
Anandtech benches SSDs pretrty often, and their reviews have a nice collection of data points there for you to see how different SSDs compare.
OCZ Trion 150 review
PNY CS1311 review
tl;dr:PNY CS1311 is the best available in your buddget. Check out Anandtech's reviews to see a good comparision of available SSDs. If you can save up a bit more, you're gonna see much better results.
Edit: I wanted to clarify when I say that the Trion 150 and CS1311 have bad random performance, I don't mean that compared to HDDs. You'll still see noticeable improvement moving from your HDD to an SSD. I am comparing those SSDs to better not-so-budget SSDs.