They are right to fear...
The first time each turn you purge virus counters, gain 4credit.
When you score this agenda, you may purge virus counters.
No MWL Entries for this card.
Can the Corp purge virus counters even if there are none in play?
Yes. Per rule 10.1.1a, the Corp can always use an effect that purges virus counters, even if that effect would not normally be determined to have the potential to change the game state.
When the Corp scores Cyberdex Sandbox, does its second ability allow them to trigger its first ability and gain 4[c]?
Yes. The first ability is active as soon as Cyberdex Sandbox enters the score area, well before the second ability resolves.
They are right to fear...
Cyberdex Sandbox is the source of the Cyberdex trick. Once score, you will never click for credits. You'll purge, giving you 4 credits for 3 clicks. Additionnaly, any purges will give you 4 creds. Cyberdex Virus Suite become a trap giving you creds. Reverse Infection become better than Hedge Fund. Macrophage has the potential to give 4 creds at every runs. Pretty good.
On top to this, it become the ultimate viruses hate. Purging become something you want to do.
It gets better. This is a neutral agenda. Cyberdex Virus Suite and Macrophage are also neutral. Which mean you can pull the Cyberdex trick with any ID. And if you use Jeeves, you can purge, gain 4 creds and still have 1 click to do something. How much crazy can this get!
Furthermore, if you 2 (or even 3) Cyberdex Sandbox scored, purging will give you more creds than any burst econ card. I-N-S-A-N-E.
The only thing that is keeping this agenda from being more widely used is the 2/4 part. 1/3 and 2/3 agendas can be scored in a turn or even from hand. 3/5 agendas are just more efficient in term of ratio click to points. 2/4 are the worst of the bunch (I'm not talking about 6/9 and 2/5 here, they are specials). 2/4 agendas require special care, because you cannot score them easily and they do not protect themselves like 3/5 agendas.
But what Cyberdex Sandbox lack in self protection is more than made up by powering up the corp. If you build you deck around Cyberdex, Aumakua and viruses using program (Pelangi, Yusuf) become useless. Freedom Khumalo become an almost blank ID. Program that get trashed by purge (like Tapwrm) become impossible to profit. Because Purging is design to cost the corp their turn. But now, you also gain creds from it. It is good enough that you might be purging even without any viruses on the board. To top it off, once you score Cyberdex Sandbox, you gain back all the credits used to score it. That is a lot of tempo.
This is an incredible agenda. It shutdown so many runner plans and speed up the corp. At top tier agenda, in my opinion. Also, I really like orcas (killer whales).