Is there any way to check the phone before buying a new device
To start with, I don't think it's possible to somehow change chip on a phone. It would stop working because of incompatible drivers.

Price of the imported phone can be lower, for example Nexus devices are 1/2 more expensive in my country than in the US and it could be used too. Anyways you should check, if it'll work on your carrier if it's imported.

Finally, there are many benchmarks to choose from. I suggest using AnTuTu. It's been here a long time and is still one of the best. It really tests your device, cause I've used it on different ROMs on one device and I had different results.