1. you don't buy the whole user base cause after what happened they have loose trust to them so you don't known what % will come back, except for withdrawing their funds.
This is a risk point, but it can be greatly mitigated with the proper communication strategy. We must email every users, put a disclaimer at first relogin and make sure there is ample news coverage about the fact BTER has been restructured and is now operating under a new paradigm whereby all important decisions are being peer-reviewed and approved by delegates elected by shareholders, and cold wallets funds are held in escrow under control of a trustless decentralized network. Users who understand that it has become technically impossible to steel the cold wallet funds or steer the company out of the right path will stay and spead the word. There will be some departures, but most customers will probably come back once the dust settles.
2. What if tomorrow they give you enough prove that it was an inside job? Will you continue to want to buy them out?
I doubt that the theft was done by the founders or at least not in a concerted manner for one simple reason: the BTER brand and business is worth more than the 7170 BTC that were stolen. If it's an insider job, it's more likely the deed of an isolated person, either a single founder with significantly less than 7170 BTC worth of BTER shares, or someone from the staff. That makes a big difference because in one case the company is corrupt, while in the other case the company is honest but has a black sheep in its ranks.
So to answer your question, if there is someday enough proof of what happened, and it's indeed a black sheep doing the job unbeknownst from the rest of the company, that person will be prosecuted and the company will be able to continue operating normally.
Now, it's also possible that BTER hasn't said everything and that the situation was already critical before the alledged hack (undisclosed earlier losses, bad trading decisions with customer funds, running on fractional reserve etc..). In that case, running with the funds could be a concerted decision from the founders given that the company was already virtually bankrupt. If that's the case though, there will be a glaring mismatch between customer balances and available funds when we inspect their finances, so obviously we won't proceed with the acquisition. Actually in that case, I would expect that BTER will refuse the bailout in first place as this would get them exposed.
3.After you buy them, how can you be sure that you get not hacked again ?
See point one. The new BTER will be using multisig coldstorage requiring signatures from delegates of the BTER DAC as well as at least one signature from a BTER staff. With that setup, the delegates would need to collude with the operational staff to steal the funds and the identity of the culprits would be know at the time they broadcast the transaction. In other words, it will be virtually impossible to steal the funds and very possible to identify and prosecute the guilty parties should that very unlikely case arise.
tl;dr: we don't really know what happened so we will need to be very vigilent when we do due diligence and ensure that BTER has full reserves for the customer funds, except for the missing Bitcoins. If there is a blacksheep in the ranks and he doesn't get nailed in the aftermath of the hack, he won't be able to reoffend anyway since going forward cold wallets will be controlled by the DAC.