Can i purchase any wordpress template and change it to my requirements?
Yes you can change the themw by your requirements.
Suggestion: if you will want to upgrade the theme in the future(if the seller will release updates with new features or bug fixes) I suggest you to start with a child theme. Most themes from themeforest already comes with child themes.
This way you will be able to update the parent theme without losing your changes.
Bear in mind also…
you can’t edit a theme then sell it on envato or other marketplaces
you need one license (purchased copy) of the theme for each individual website that you use it on.
So if I purchase one theme and modify the original coding, not creating a child theme. I could then sell it on as my own? But it would be upto me to update and maintain?
You didn’t said anything about reselling the theme! No, one thing is clear you will not be able to resell that theme on any other marketplace.
But if the theme will be for one customer(for an individual client/website) then you can modify it and install it on that site.
As @charlie4282 already mentioned you need one license (purchased copy) of the theme for each individual website that you use it on.
Sorry I miss read the: you can’t sell it on.
But I can still purchase one and make changes to the original coding for one website?
Yes! One license per one website. With that theme you can do whatever you want in terms of changes in that one website
I suggest you read this https://themeforest.net/licenses/standard
You can buy and edit it to your hearts content BUT what you can then do with that edited theme or website is restricted based on the license you buy and type of modification you make.
Okay thanks that’s cleared things up for me.