Hi, welcome to phpBB.com.
1. phpBB should behave fine with both.
2. For your 3.0.8 board, you'd need to upgrade to 3.1.x, which is relatively easy. Exact instructions on how to do this can be found in our upgrading section in the User Guide, over here
3. For that you'd need to do a conversion, exact instructions for upgrading that are these:
Information on how to convert from phpBB2 to phpBB3 is here
Basically, you must have both phpBB2 and phpBB3 installed on the same server. After you have finished installing phpBB3 you will have the option to convert your phpBB2 board using the built in converter. The information it asks you for is related to the phpBB2 installation and can be found in its config.php file on the server. All of your users, posts, forums, PMs etc are converted. However, styles and MODs are not (with the exception of the birthday and attachment MODs which are supported by the converter). The phpBB2 database is not converted per se
, it is merely read and converted/moved to the new phpBB3 database, so if it goes wrong your old board will not be affected at all. At the end of the conversion, once you’re happy it’s all working, your phpBB2 database and files can be backed up and removed if you so wish. They are not connected to the new installation in any way.
4. Merging installations is generally very difficult. I'd recommend you upgrade both boards to the latest version of phpBB (for security reasons) and then use of them to continue using and the other one as an archive for users to look up old topics/posts if they wish to do so.
5. According to our board rules, it's not allowed to solicit people for your own (paid) private work openly on the board. We would be more than willing to help you go through this process yourselves if you post a topic in the support forums with your questions. If you after that still intend to let somebody else to the work, we usually recommend you look at signatures of team or community members here on the board. Many of them including a line allowing you to contact them for private work.