Oresteia, all you have to do is to install DBW (Dark Blue World). Since you have UP3, it is easy to install DBW on top of it, and DBW already includes an improved version of Ar-234 C-2 and many other improvements. DBW also has all needed files like air.ini easily available for you if you want to install more planes. DBW will also be the best supported base for new mods from now on for offline players and will develop quickly (for example it will get this week almost all maps ever released).
You can still use UP3 for online play, but online you won't need planes that are not in UP3. DBW won't damage your UP3 and will give a whole new experience for offline play.