Just three questions: Could I cut the Nobz for more Boyz / or include a Painboy?
Well, you could, but in the end the game is won by holding objectives and a second unit will allow you to be in two places at once, while a painboy would be forced to run with the boyz. When increasing your army, I would not hold onto them, but keep in mind that a pain boy is rarely worth his points unless you manage to keep two or more units of boyz near him.
Why did you select these powers for the Weirdboy and not something like da Krunch / da Jump?
In my experience when playing on small boards, there are few good places to actually put down a unit with da jump. In addition, if you don't have a lot of targets to jump, as you can't jump away your character's body guards. Da Krunch might sound like an awesome power, but it fails to impress when used regularly, especially when you consider that your weird boyz can always cast smite. Feel free to give it a try yourself though, it's not completely horrible and you won't learn without getting a bloody nose here and there.
On the other hand, warpath works well on any of your units, including the boss and the dread, while seizures can neuter armies which try to get leverage out of expensive units which people struggle to beat at low point costs.
And what would be your suggestion for future purchases to enlarge my little ork horde?
Depends. Orks work well as mechanized force OR as infantry horde. The two styles never have and probably never will mix well, so you would have to decide which way you want to go first.
For mechanized orks you would dip into a Mad Max style army with lots of buggies and potentially some stompy robots, the horde force would lean towards boyz, Ghazghkull Thrakka (currently hard to buy).
If you are undecided, a few units like MANz
, mek guns or burna bommers currently do well in any kind of army.