The long form of the answer:
Online mining is that grey area where you don't invest until you're 100% sure. The aforementioned website is likely to scam you if they are providing such a service in the first place.
To address the question, how much will it take to launch a 51% attack;
Firstly you can check the current compute power (hash rate) and it's $ equivalent.
http://rindex.io/attack-cost gives you the exact cost for a 51% attack at any given point of time.
If you compare the calculated hashrate against one of the more popular ASICs on the market, the figure stands at about 2.2 billion. However, this assumes that suddenly half of the network right now goes rogue and decides to attack it. If not, you would need more than 2.2 billion dollars since the hashrate needed has to be 51% of the total hashrate (after the attack starts).
Its rather resource consuming and it isn't that big of a problem since none of the government really cares that much to spend their budget on attacking a cryptocurrency. You probably wouldn't profit all that much when you attack; doubt exchanges credits more than 2 billion dollars worth of Bitcoin automatically without raising a red flag.
Also, If you don't have 51% of the whole network (ie. more hashrate than the others), you don't have a 100% chance of executing an attack. The honest/longest chain still has a chance of overtaking you and you have to acknowledge their blocks. The probability of success decreases with the number of blocks.
Assuming the attacker decides to do 51% attack by purchase lots of ASIC/GPU, but it's not a really accurate number because:
There's no way you could get thousands (maybe millions in the coming future) of ASIC/GPU units easily
Even if they could, it's obvious that the price of ASIC/GPU will be more expensive due to high demand and low supply (this is already the case with graphics cards, poor gamers!)
Unless you have a connection, it's highly unlikely you could use lots of electricity and big mining location without ringing any alarms
Now, what if the attacker wants to hire a pool instead of investing in purchasing the equipment. In the current state where almost all mining power is concentrated within less than ten pools, that's enough to 'buy' a pool admin(s).
Why would even mining pools agree to "rent" their hashpower? If they actually did that, they would quickly lose their users. I don't really think if there would be many farms interested in this form of attack. Long-term earning is much wiser than trying to cheat and sell as much as possible. The rest of the network would quickly realize that the 51% attack was performed. Just look what is going on with Verge which is/was being attacked. The 51% attack is the reason why there are people who think that banning ASICs and mining only with GPUs is a good way to prevent it from happening.