Questions about Cs:Go/Steam sales

I'm wanting to buy an account to smurf on and seen pretty good offers as of right now, some with pretty insane prices and some for reasonably cheap but suspicious prices.
A few questions,
If I do buy an account, can it be recovered?
What would I have to be provided with and can I change everything to my own such as Email,Phone for prime, etc?
If the account that's for sale has prime meaning it's obviously linked up to a phone, what can I do there in order for me to have it linked to my device?
What are X digit accounts? Seen titles such as "6 digit accounts for sale". Honestly unsure of what it is.

That'll be all for the moment, If anyone wishes to contact me reach my skype: syntaxxgaming
Thank you to anyone who answers my questions in advance.

 

It can be recovered
Yes you can change prime phone-remove, change email etc
6 digit is like this STEAM_0:1:123456

 

Well since it can be recovered that slims down the chances of me buying one. If anyone who's experienced at buying/selling accounts can contact me i would appreciate it.