Join a pool or create my own pool with my 500 PCs? I'm an internet provider for a village that doesn't have access to internet except through my service. 

I thought about using my customers' PCs for mining and splitting the profits between us.

Each computer will vary from 50-100kh/s. So it will lead to more than 25000khs/s if combined in a pool mining.

Should I set up each computer to do SOLO mining, then each one sends me the blocks it mined, and I exchange them to USD and split the profits?

OR

Should I join a mining pool ex. http://www.wemineltc.com/ and add the computers as workers under my account name?

OR

Should I create my own pool and make them search together to find the blocks and have the blocks go directly to me?

If the 3rd option is the best, are there any simple tutorials on how to do it with the GUIminer Scrypt Alpha (so that the users don't have to be confused with the command lines stuff, and to make it automated with Windows startup)?
With that much hash power, I believe you could be better off creating your own mini-pool and putting those computers in it. In theory, solo mining will always give you more coins in the long term for pools always take a fee for their services. In practice, of course, you could be mining for years without ever finding a block, so it's more a matter of how much you can wait to see a return on your costs and investment: more constant payments but less gain in the long term, or the inverse.

This formula can aproximate how much time on average would elapse before you found a block - it depended on the difficulty level of the network and your hashing power. Have a look at it and decide if you can wait that much weeks/months with no yield in order to gain more in the end.

https://en.bitcoin.it/wiki/Difficulty#How_soon_might_I_expect_to_generate_a_block.3F