To find out whether a new version of your message performs better, you first need to know how it has performed in the past. Establish a baseline to compare the results of your new message to. Be sure to note any relevant results like amount of recipients, time of day, day of week, time of month (e.g. did you send the message around pay-day or just before, when everyone postpones spending money until their pay check arrives?), open rate, click-through-rate (CTR), amount of conversions (note: specify what a conversion is for you), conversion rate, and anything else of importance.  
The answer to this question is complicated. You can legally purchase phone numbers, since there is no law that forbids you to do this. When you are buying phone numbers, you are essentially purchasing data. However, it is what you do with those purchased phone numbers that determines if you stay on the legal side of the law or not. Read on to find out more about what you can or cannot do with purchased phone numbers.
This question exists for many. First, NEVER EVER purchase a list of mobile numbers. (SMS/MMS) marketing is permission based, and ultimately you want a list of customers numbers who are genuinely interested in your product or service and are happy to receive your text message and are likely to buy your goods and service. Remember, you can turn customers off and possibly upset a lot of your potential customers when sending text messages on their mobile devices without their consent.