Feel free to pay them the way you like it. I'm sure the possibility of a chargeback is some additional security, but I've used BTC to invest into it.

About the other problems: They may have some technical gliches but are very very friendly. Some arguments which convinced me:

• They offer to show you their ID-scans and personal data when you want this
• They showed details of their order of the KnCs
• Their whole story looks legit to me and they are currently offering to a small circle already some miners (ask them for a proof, if you are unsure)
• If they would scam, they wouldn't invest time/ressources into their website (which is ready for real use, what you can see when your account is activated)
• They have replied to me with real names sometimes
• They are not offshore, they live in Scandinavia and KnC is produced there too
• They offer a refund if KnC delivers not till the end of march
• They offer a general 14 days money back guarantee
• They take a 10% fee from the payouts which makes only sense if they payout

There isn't any 100% proof for nearly anything. But for me this is enough. As I said, I personaly think the worst what could happen with this guys would be if they have to refund because KnC doesn't deliver. Try to see it from their view too: What else could they do to proof that they are legit?

If the machines have more than 3 TH I doesn't care as long as I and people who found them via me get what we have on our contract. They are clearly defining the borders. If there are getting more than 3 TH, they could be perhaps extremly  generous and reduce the 10% fee (you have to ask them).

Addtitionaly (again): Use this calc http://btcinvest.net/en/bitcoin-mining-profit-calculator.php?diff=2193847870&dcosts=1200&diff_mincrease=30&blpbtc=25&dhsmhs=300000&diff_mincreasedecrease=3&btcusd=849.32&dpowcon=0&btcusd_mincrease=1&pcost=0&calcweeks=32&dleadtime=4&action=calc to calculate what it means when they have delays or ajust it according your predicitions and decide then.