So guys, it's done.
Epoch is up to date with 1.0.5.1 and OVERPOCH is installed.

Do the following to join the server:

Upgrade Arma2 to Beta 112555.
Install Overwatch via DayZ Commander to the latest version.
Update Epoch to the latest version.
Add in DayZ Commander under Settings/Additional Launch Parameters: -mod=@DayZOverwatch;@DayZ_Epoch;@DayZEpochServer

There are still things to do:

They changed the type of the loot tables with the newest patch, so the ones for Overpoch weren't working anymore and I wasn't able to join.
Took me some time to figure this out.
The consequence is that the loot tables for the zombies are not working atm. They'll drop nothing.

Second thing is that I have to add the Overwatch stuff, the new weapons and cars to a trader. This needs some preparations, the adding itself is simple then. Guess I will do that tomorrow.

You will find new weapons and ammo in the buildings. Some buildings, the hospital e.a. can be entered now. Looks nice.

The new Chopper CH57 is already added to the army trader (hero)

Please check if the scripts are all working.
Did a nightshift yesterday and I'm not sure if I readded all in the right way. Also I didn't test them yet.

I removed the building scripts to the old place to prevent the error message when joining. I had some problems again with some uneven buildings, but last time all seemed to be fine again.

Let me know if you remember any errors.

In an hour I have to leave for a date, so maybe I'm late in the evening online again.

Have fun!

I had some problems with this issue on my test runs.
It happens when a script isn't loaded correct.
But I fixed that before I opened the server and actually I joined on first try.

I have actually 3 new players on the server and that's over 20 minutes now without issues.
I am online since 2 hours now and connected several times.
You're sure you installed all listed above and copied and pasted the correct launch parameters?

EDIT: One of them had the same issue after a while. I deleted his actual life and he could join again.
So, I deleted also the lifes of Gareth and DavidoSS so far. Hope that this is of help.
It seems that only some players having this problem. Maybe changing the mods with the old database data of some players causing this.

Still unclear, why some of you having problems with joining.
Some join after 2-3 tries, myself and some others join without problems.
Will try to fix that of course...

It's definitely the loot tables causing these problems.
I changed them again and they're working fine now, except zombies still spawning no loot.
Need some more research on this, but somehow I like it not to get all that stuff on the fly...