which site are you using for renting ? is it some kind a auction ? i looked nicehash and it was very complicated. i read some users comments and they were saying they invested (for example) 1k usd but they get only 100$.
Nicehash is actually very easy and safe to use. I've never had any problems with it. Yes, you can rent hash power rather than individual mining rigs there. It is very comfortable. You can pay like 0.02 BTC and get thousands of coins, because you mine at several Gigahash per second and when you mine those new coins, the difficulty is still low. Hold those. I only mine coins that make a decent impression, my standards are very high.