These are the one's Scotty recommends (look under Tools at the top right). I would add a compression tester, as that gives you a definitive measure of engine health.

https://www.amazon.com/shop/scottykilmer?listId=2XW7WGTYW92WE

Check out Autel's stuff. Excellent scanners.

What are you trying to do with the scan tool? You need to figure that out first.  Otherwise, for most people, a basic scan tool is all they need, and for that the Bluedriver is excellent (and works with your smart phone).  It is about $100.  I would buy direct from Bluedriver to make sure you are getting the latest firmware and not getting scammed (e.g. clones, etc).

https://www.bluedriver.com/products/bluedriver-scan-tool

Good advice (as cited above, with the link). I have a Blue Driver. I find it useful, even in terms of checking my car's current health. (The Blue Drive does Enhanced/Mode 6 data probing, which is manufacturer specific, for many models). 

Apropos testing a used car, you might want to also consider buying a Block Tester ($35 or so), to test for compression gases in the radiator (that is, for a head gasket leak). It's an easy test, unobtrusive, relatively fast and IMHO, necessary.

One additional caveat about many scan tools: All bets are off if you're looking at a hybrid (which is not recommended). That's an entirely different scan beast.

Does anyone have any experience with a Carly scan tool?

If you find yourself needing a more advanced scan tool (that can scan every computer module on the vehicle and want bidirectional control) I’d go with Autel.

For scanning every computer module, I strongly recommend the Autel Maxicheck MX808 which is about $400, and the cheapest tablet-style scan tool (easier on the eyes). However, it does not have bi-directional control which for most of us is fine.  If you want bidirectional control, step up to the Autel Maxisys MS906BT ($1200-$1600) - also tools Scotty Kilmer and Car Wizard use, but probably overkill for vast majority of us.

If you do buy an expensive scan tool (several hundred or more), I strongly recommend you buy from one of the company’s authorized dealers if you care about support, service, software updates, and warranty, and to make sure you don’t get scammed.  Some of those expensive ones from Amazon are not North American models and if you need to have it serviced you have to send to China!  And Amazon is NOT an authorized dealer for Autel.

link to tool: https://www.autel.com/mk2/3223.jhtml

One place I recommend buying from:  they are an authorized Autel dealer

https://www.aeswave.com/MaxiCheck-mx808-Scan-Tool-p9590.html