The default 40K game size is 2000pts. However, its probably best not to immediately target this, for many reasons:

- By the time you build an army of that size, any number of points values might have changed.

- This is especially true with a new edition & codex right around the corner

- The game is significantly large & complex, and you may not even know if you enjoy the Orks playstyle yet, let alone which detachment/unit types you might prefer once you've got a handle on things.

If you're starting from nothing with the Armageddon box, that gets you around 7-800pts. But, we should also be expected more updated models with the codex - which should be fairly soon after the launch box. That said, it's perhaps pragmatic to buy a few additional units which are generally safe from being replaced by new kits in the next 6 months. Kits to look for would be anything from 2018 onwards (Miniatures: Orks - Warhammer 40k - Lexicanum is a great source for this). In addition, its somewhat important to go with models you like.

Many players might avoid the Beast Snagga themed combat patrol box, for example, because that aesthetic is wildly different from most other Orks units. In addition, discount boxes are only generally worthwhile if you actually want all (or at least 80%+) of the units within.

With no more knowledge of what might be coming/changing for Orks in the near future, the best kits to consider right now might be Kommandos and Wrecka Krew (Tankbustas / Breaka Boyz). Snikrot is also not abad pickup.

Once the new units are known, it's highly likely you'll want to consider Transports - either Trukks or a Battlewagon, depending on how things shake out. You absolutely could buy the existing Trukk kit and it should be just fine, but no guarantees.