I need to set up a self-contained Hotspot, with preferably Hotspot and User Manager on the same RB which also has to include wireless, which RB will have enough Memory, Ram to realize this.
Am I correct in assuming that I need at least a RB433 ?

Minimum you need a device with a level4 license so the wireless card can act as an AP. A 433 is a good place to start if you need the Ethernet interfaces.

With that being said, User Manager takes up a lot of resources, so the exact board that you need really comes down to what you want it to be able to do. How many people are going to be on the hotspot, total bandwidth, queues, firewall rules, etc.

Also if you plan on having several hotspots, now is the time to plan on getting a centralized authentication system in place. Administering a local authentication system if you only have very few locations is all fine and good, but as soon as you get enough of them it will become a massive nightmare.

Thanks for the Answer, Is my assumption correct that I can only use a Board with a Memory Card or USB for the necessary Storage ?

good information

The on board NAND should be enough to hold User Manager etc especially for just one network. Most boards come with at least a 64Mb NAND I want to say.

You of course can add another store for extra storage if you wanted to run a Proxy off of the box or store local logs if the board supports it, I just wouldn't use the proxy more for than a small network. Also the router board does not have a flash controller so every time it writes or reads from the flash it eats up CPU cycles. The Dude or running a proxy off of the box generally uses the the flash the most.

Will the NAND Flash be enough, as the User Manager Wiki talks about "The router should have at least 32MB RAM and 2MB free HDD space." ?

HDD space is the NAND, that's the flash memory of the routerboard. You should have around 50% free space on it after routerOS is installed, so yes plenty of space.