Do you mind explaining what changed to the bad with DR from 2->3 that you say it is only a shell now? I only saw screenshots and some gameplay videos of the third and it looks like mindless fun. I never played any DR before that and with DR2+OTR just having had sales, I thought about picking OTR up once it's >5 bucks. Hope the region-lock for Germany is not that strict though. I read here that during sale there were changes, so I am confused a bit.
I could honestly write a whole essay about its shortcomings, so I'll attempt to keep it concise. It's as you say: it's mindless. A superficial imitation of the older entries to the series. In your position I'd strongly recommend playing DR2 first unless you have an Xbox 360; the first Dead Rising is still a unique experience that does not compromise when gunning for the true ending, but it's very replayable thanks to progress carrying over if you want to reset the storyline and it rewards exploration. Heavy on the time management too, which (to me) makes for refreshing gameplay. Dead Rising 2 by-and-large still adheres to the same principles while smoothing out some rough edges and lowering the difficulty a little, but it's still challenging enough while in retrospect serving as a competent middleground between 1 and whatever 3 tried to do.
Here's my problem with 3: it rips out everything that distinguishes the franchise from the pack to this day... or rather, used to. It's filled to the brim with Ubisoft carrot-on-a-stick collectibles or mindnumbing fetchquest missions, both of which end up being meaningless in the grand scheme of it all. It removes any semblance of exploration by pinpointing every little tidbit on the mini-map while bombarding you with further notifications on the HUD, while the lockers inside the many safehouses trivialize this aspect (among others) even further. Combo weapons and the skill tree make the player so insanely overpowered that you never feel under any threat or pressure; time is no longer an enemy and bosses (although nowhere near as finnicky in 3) are total push-overs. Survivors also no longer fulfill any worthwhile purpose; they admittedly had shitty A.I. previously, but them being so weak added an appropriately stressful layer and (without spoiling the surprise elements) the developers provided them some extra flavor on a mechanical level in prior iterations. The story in 3 is self-serious trite with constant tonal shifts and your protagonist lacks backbone: Frank West and Chuck Greene were charismatic men on a mission in 1 and 2 respectively, whereas Nick in 3 gets bossed around and yelled at like a defenseless puppy. The game goes out of its way to delay your movement at every passing wake, be it with endless back-and-forths that lack context, with driving vehicles feeling like a chore or with the harmless zombie blockades grabbing you a ton... EVEN WHEN SLICED IN HALF.
The list goes on and on. It's not far off from being a third-person Dead Island: overly reliant on busybody work, utterly boring and undercooked. There's cheaper zombie games that run better if you want to dick around killing zeds plus Dead Rising 1 and 2 are executed far better.