Hi Sebbb, and welcome to AF
I'm typing this on a mobile app, so can't see your post at the same time, so apologies if I miss any points!
If updates are your concern then you don't need to worry with the Nexus 4 if you buy it from the Play Store. That's straight from Google, with nobody else in the chain to slow the update.
I don't know what apps you tried, but I've very rarely had problems with stability on my old HTC so suspect the cheap tablet. The processor used by the Nexus is going to be in a lot of high end devices in the near future, and the screen resolution is quite standard, so again I don't think you'll have compatibility problems with it.
As for AV, the companies see a market, but that doesn't mean it's needed. Some people like to have a scanner for peace of mind, some like other security features that come with some of these apps (phone location, remote wipe, etc), most don't use one. There is malware for any platform, but if you take reasonable care with what you install and where you install from you will be very unlucky if you encounter it. At time of writing there are no android viruses in the traditional PC sense.