Basically you can use both the x405p and x406p for your purposes. The "mechanical" difference is the the CI-slot and missing RS-232 input on the backside (it can be fitted internally on the inside of the x406p however. But be aware you loose the guarantee once opening the cover, it is labelled with a sticker.) But this is not the important point.
The real reason is the emu software. There has been a major change in production units about a year ago, and only the x406p came back with new and improved emu software. The x405p had been forgotten. When it finally came there where some incombatibilities with older units of the x405p. So, to answer your other question: the firmwares are not exactly the same. Settings, softcam.key and serverdata can be used in both units. Just add the 3 € extra and I asure you you will have no problems.
I will not write a complete describtion of the conversion process here (or even a pdf) with all do's and dont's but this is what you will have to do:
1.) Download the convert software via USB
2.) Download the emu-software via USB
3.) Change in System Settings "Subtitles language" to "undefined"
4.) Change in time settings time zone to "Berlin" and summertime to "auto"
5.) Make sure via F1 + 0 + 0 + 0: "patch enabled"
6.) Allow internet access for cs by F1 + 1 + 1 + 1 and in new box click nr. 5 "Internet activation"
7.) Download settingslist if available or make your own one after full scan
8.) Eventually run "G-NET-Update", this will give you some free key updates
9.) Upload Serverdata, prepared previously by x406p server tool
10.) Reboot your machine and have fun!
All necessary files you find here in database.
Ask your further questions, once you have your machine and I will help you until success.